# Import and interpolate data with pgfplots (and a solution for plotting the chi square distribution)

I have to draw a chi-square distribution, and this is quite a challenge with pgfplots. I found some solutions but I discarded them all for different reasons and I choose the lazy solution:

1. I generated the curve using Octave/MATLAB, then I saved the points in a CSV file (downloadable here):

x = .1:.1:8;
pdf1 = chi2pdf(x,1);
pdf2 = chi2pdf(x,2);
m = [x' pdf1' pdf2' pdf3' pdf4' pdf5' pdf6' pdf7' pdf8'];
csvwrite ('chisquare.csv', m);

2. I imported them in LaTeX using \pgfplotstableread[col sep=comma]{chisquare.csv}\dataChiSquare:

This is the plot of the chi-square distributions for K=1, ..., 8:

 \begin{tikzpicture}
\begin{axis}[%
no markers,
height=5cm, width=10cm,
smooth,
axis x line=bottom, axis y line=left,
xtick=\empty, ytick=\empty,
clip=false,
enlargelimits=upper,
restrict x to domain = 0:7,
restrict y to domain = 0:0.8,
]
\addplot table[x = x, y = pdf1] from \dataChiSquare;
\addplot table[x = x, y = pdf2] from \dataChiSquare;
% .........
\addplot table[x = x, y = pdf8] from \dataChiSquare;
\end{axis}
\end{tikzpicture}


My question is: how can I compute points of the plots, as I asked here? Is there any way to (easily) define a function that interpolates the points using tikz or pgfplots? I need to draw some vertical lines from x axis to the plot, but I also need the coordinates for other purposes.

The preamble of my other question can also be used to have a MWE for this one.

Thank you!

• You could get the coordinates from intersections. My problem is that I do not have MatLab, so I cannot create the .csv files. Any chance you want to post a really minimal compilable example that people (and ducks and marmots) can play with?
– user121799
Jun 18, 2018 at 2:45
• I added the link to the .csv file on my Dropbox. I can't check right now if the code is compilable, however it should be exactly the same code in my other question linked above, except for the tikzpicture that has to be replaced with the one in this question Jun 18, 2018 at 2:53
• PS: @marmot thank you for helping with all of my pgf questions! Jun 18, 2018 at 2:55

Here is an example using intersections and calc. The y value is stored in \yQ. I just made up some data but it should work with your data as well. (I could not use \pgfplotspointgetcoordinates because of expansion issues but it is straightforward to get the coordinate with calc.)

\documentclass[tikz,border=3.14mm]{standalone}
\usepackage{filecontents}
\begin{filecontents*}{chisquare.csv}
x,pdf1,pdf2,pdf3,pdf4,pdf5,pdf6,pdf7,pdf8
0,1000,0.5,0,0,0,0,0,0
0.1,1.200038948430136,0.475614712250357,0.1200038948430136,0.02378073561251787,0.004000129828100455,0.0005945183903129469,8.000259656200907e-005,9.908639838549101e-006
0.2,0.8071711293576808,0.4524187090179798,0.1614342258715362,0.04524187090179798,0.01076228172476909,0.0022620935450899,0.0004304912689907634,7.540311816966337e-005
0.3,0.6269100992275207,0.4303539882125289,0.1880730297682562,0.06455309823187934,0.01880730297682562,0.004841482367390948,0.001128438178609537,0.0002420741183695475
0.4,0.5164415474672782,0.4093653765389909,0.2065766189869113,0.08187307530779819,0.02754354919825484,0.008187307530779821,0.002203483935860388,0.0005458205020519878
0.5,0.4393912894677224,0.3894003915357024,0.2196956447338612,0.09735009788392562,0.03661594078897687,0.0121687622354907,0.003661594078897689,0.001014063519624225
0.6000000000000001,0.381545289384093,0.3704091103408589,0.2289271736304558,0.1111227331022577,0.04578543472609117,0.01666840996533865,0.005494252167130943,0.001666840996533865
0.7000000000000001,0.3360144677267703,0.3523440448593567,0.2352101274087392,0.1233204157007749,0.05488236306203916,0.02158107274763561,0.007683530828685487,0.00251779182055749
0.8,0.298983539918205,0.3351600230178197,0.2391868319345639,0.1340640092071279,0.06378315518255041,0.02681280184142558,0.01020530482920807,0.003575040245523413
0.9,0.268136721052083,0.3188140758108867,0.2413230489468747,0.143466334114899,0.07239691468406238,0.03227992517585227,0.01303144464313124,0.004841988776377838
1,0.2419707245191433,0.3032653298563167,0.2419707245191434,0.1516326649281584,0.08065690817304777,0.0379081662320396,0.01613138163460956,0.006318027705339933
1.1,0.2194581724133437,0.2884749051902433,0.2414039896546781,0.1586611978546339,0.08851479620671532,0.04363182941002433,0.01947325516547737,0.007999168725171124
1.2,0.1998677639017332,0.2744058180470132,0.2398413166820799,0.1646434908282079,0.09593652667283198,0.04939304724846241,0.02302476640147969,0.009878609449692479
1.3,0.1826614817951091,0.261022888380508,0.2374599263336419,0.1696648774473302,0.1028993014112448,0.05514108517038233,0.02675381836692366,0.01194723512024951
1.4,0.1674325573450835,0.2482926518957047,0.2344055802831169,0.1738048563269934,0.1093892707987879,0.06083169971444766,0.03062899582366061,0.01419406326670446
1.5,0.1538663228054553,0.2361832763705073,0.2307994842081829,0.1771374572778805,0.1153997421040915,0.06642654647920521,0.03461992263122744,0.0166066366198013
1.6,0.1417145653062239,0.2246644820586108,0.2267433044899583,0.1797315856468887,0.1209297623946444,0.0718926342587555,0.03869752396628622,0.01917136913566813
1.7,0.1307781819238881,0.2137074659743633,0.2223229092706098,0.1816513460782088,0.1259829819200122,0.07720182208323877,0.04283421385280416,0.02187384959025099
1.8,0.1208951224732049,0.2032848298702996,0.2176112204517688,0.1829563468832696,0.1305667322710613,0.08233035609747134,0.04700402361758206,0.02469910682924141
1.9,0.1119318050861699,0.1933705117272506,0.2126704296637229,0.1837019861408881,0.1346912721203578,0.08725844341692184,0.05118268340573599,0.02763184041535858
2,0.1037768743551487,0.1839397205857212,0.2075537487102974,0.1839397205857211,0.1383691658068649,0.09196986029286061,0.05534766632274598,0.03065662009762019
2.1,0.09633657731357953,0.1749688745555777,0.2023068123585171,0.1837173182833566,0.1416147686509619,0.09645159209876222,0.059478202833404,0.03375805723456678
2.2,0.08953128037314291,0.1664355418490398,0.1969688168209144,0.1830790960339438,0.1444437990020039,0.1006935028186691,0.06355527156088171,0.03692095103351201
2.3,0.0832928061175755,0.1583183846895266,0.1915734540704237,0.1820661423929556,0.1468729814539915,0.1046880318759495,0.06756157146883611,0.04013041221911396
2.4,0.07756236924025954,0.150597105956101,0.1861496861766229,0.1807165271473213,0.1489197489412983,0.1084299162883928,0.07148147949182322,0.04337196651535711
2.5,0.07228895706727252,0.143252398430095,0.1807223926681813,0.1790654980376188,0.1506019938901511,0.1119159362735118,0.07530099694507554,0.04663164011396324
2.6,0.06742804459323157,0.1362658965170063,0.1753129159424021,0.1771456654721082,0.1519378604834151,0.1151446825568703,0.07900768745137589,0.04989602910797716
2.7,0.06294056442554451,0.1296201303229458,0.1699395239489702,0.1749871759359768,0.1529455715540732,0.1181163437567843,0.08259060863919951,0.05315235469055297
2.8,0.05879207325254447,0.1232984819708032,0.1646178051071245,0.1726178747591245,0.1536432847666496,0.1208325123313872,0.08604023946932379,0.05638850575464736
2.9,0.05495207145796054,0.1172851440468988,0.1593610072280856,0.1700634588680033,0.1540489736538161,0.1232960076793024,0.08934840471921335,0.0595930703783295
3,0.05139344326792309,0.1115650800742149,0.1541803298037693,0.1673476201113224,0.1541803298037693,0.1255107150834918,0.0925081978822616,0.0627553575417459
3.1,0.0480919926380412,0.1061239869133715,0.1490851771779277,0.1644921797157259,0.1540546830838587,0.1274814392796876,0.09551390351199233,0.06586541029450528
3.2,0.04502605584019208,0.1009482589973277,0.1440833786886146,0.1615172143957243,0.1536889372678556,0.1292137715165795,0.09836091985142761,0.06891401147550903
3.3,0.04217617598652614,0.09602495431037704,0.1391813807555363,0.1584411746121221,0.1530995188310899,0.1307139690550008,0.1010456824285194,0.07189268298025039
3.4,0.03952482794294565,0.0913417620263673,0.1343844150060152,0.1552809954448245,0.1523023370068173,0.1319888461281008,0.1035655891646358,0.0747936794725905
3.5,0.03705618452374813,0.08688697172522257,0.1296966458331184,0.1520522005191395,0.1513127534719715,0.1330456754542471,0.1059189274303801,0.07760997734831081
3.6,0.03475591672713835,0.08264944411079327,0.125121300217698,0.1487689993994279,0.1501455602612377,0.1338920994594851,0.1081048033880911,0.08033525967569108
3.7,0.03261102221401006,0.0786185831568138,0.1206607821918372,0.1454443788401056,0.1488149647032659,0.1345360504270977,0.1101230738804168,0.08296389776337691
3.8,0.03060967735558654,0.07478430961131752,0.1163167739512288,0.1420901882615033,0.1473345803382232,0.1349856788484282,0.1119742810570497,0.0854909299373378
3.9,0.02874110905657778,0.07113703579325677,0.1120903253206534,0.1387172197968507,0.1457174229168494,0.1352492893019295,0.1136595898751425,0.08791203804625421
4,0.02699548325659403,0.06766764161830635,0.1079819330263761,0.1353352832366127,0.1439759107018348,0.1353352832366127,0.1151807285614679,0.09022352215774178
4.100000000000001,0.02536380756620646,0.06436745179390209,0.1039916110214465,0.1319532761774993,0.1421218683959769,0.1352521080819368,0.1165399320847011,0.09242227385599021
4.2,0.023837845937227,0.06122821412649095,0.1001189529363534,0.128579249665631,0.1401665341108948,0.1350082121489126,0.1177398886531517,0.0945057485042388
4.3,0.0224100436236817,0.05824207888674848,0.09636318758183131,0.1252204696065092,0.1381205688672915,0.1346120048269975,0.1187836892258708,0.09647193679268147
4.4,0.02107346097903018,0.05540157918116694,0.09272322830773279,0.1218834741985672,0.1359940681846747,0.134071821618424,0.1196747800025138,0.09831933585351095
4.5,0.01982171487060489,0.05269961228093217,0.08919771691772202,0.1185741276320974,0.1337965753765831,0.1333958935861096,0.1204169178389248,0.1000469201895822
4.600000000000001,0.01864892668496918,0.05012942186140185,0.08578506275085825,0.1152976702812243,0.1315370962179827,0.1325923208234079,0.1210141285205441,0.1016541126312795
4.7,0.01754967605644785,0.04768458110777481,0.08248347746530486,0.1120587656032708,0.1292241146956443,0.1316690495838432,0.1214706678139056,0.1031407555073439
4.800000000000001,0.01651895958214552,0.04535897664470624,0.07929100599429849,0.108861543947295,0.1268656095908776,0.130633852736754,0.1217909852072426,0.1045070821894032
4.9,0.01555215389559439,0.04314679324968525,0.07620555408841254,0.1057096434617288,0.1244690716777404,0.1294943132406179,0.1219796902441857,0.1057536891465046
5,0.01464498256192649,0.0410424993119494,0.07322491280963242,0.1026062482798735,0.1220415213493874,0.1282578103498418,0.1220415213493874,0.1068815086248682
5.100000000000001,0.01379348633346337,0.03904083300057656,0.0703467803006632,0.09955412415147029,0.1195895265111275,0.1269315082931246,0.12198131704135,0.1078917820491559
5.2,0.012993996368508,0.03713678910716694,0.06756878111624158,0.09655565167863402,0.1171192206014854,0.1255223471822242,0.1218039894255449,0.1087860342245943
5.300000000000001,0.01224311007004337,0.03532560653021478,0.06488848337122986,0.09361285730506919,0.1146363206225061,0.1240370359292167,0.1215144998598565,0.1095660484041414
5.4,0.01153766924671384,0.03360275636987488,0.06230341393225473,0.09072744219866219,0.1121461450780586,0.122482046968194,0.1211178366843032,0.1102338422713746
5.5,0.01087474033728314,0.03196393060335379,0.05981107185505726,0.0879008091592229,0.1096536317342717,0.1208636125939315,0.1206189949076989,0.1107916448777706
5.600000000000001,0.01025159647287075,0.03040503131260898,0.05740894024807616,0.08513408767530517,0.1071633551297422,0.1191877227454272,0.1200229577453113,0.1112418745623988
5.7,0.009665701179594184,0.02892216043741923,0.05509449672368685,0.08242815724664482,0.1046795437750051,0.1174601240764689,0.1193346799035058,0.1115871178726455
5.800000000000001,0.009114693548557632,0.0275116100282036,0.05286522258163426,0.07978366908179048,0.1022060969911596,0.1156863201685962,0.1185590725097452,0.1118301094963097
5.9,0.008596374721056833,0.02616985297421619,0.0507186108542353,0.07720106627393776,0.09974660134666277,0.1138715727540582,0.1177009895890621,0.1119737132081573
6,0.008108695554940244,0.02489353418393197,0.04865217332964145,0.07468060255179593,0.09730434665928291,0.1120209038276939,0.1167652159911395,0.1120209038276939
6.100000000000001,0.00764974535371112,0.02367946219557045,0.04666344665763784,0.07222235969648989,0.09488234153719693,0.1101390985371471,0.1157564566753803,0.1119747501794329
6.2,0.0072177415535363,0.0225246011967789,0.04474999763192506,0.0698262637100146,0.09248332843931178,0.1082307087505227,0.1146793272647467,0.1118383990422068
6.300000000000001,0.006811020275148059,0.02142606343352008,0.04290942773343277,0.06749209981558829,0.09010979824020883,0.1063000572095515,0.1135383457826631,0.1116150600700292
6.4,0.006428027657939708,0.02038110198918311,0.04113937701081413,0.06521952636538594,0.0877640042897368,0.1043512421846175,0.1123379254908632,0.111307991663592
6.5,0.006067311902576734,0.019387103915861,0.03943752736674877,0.06300808772654828,0.085447975961289,0.1023881425556409,0.1110823687496757,0.110920487768611
6.600000000000001,0.005727515956354745,0.01844158370062,0.03780160531194131,0.060857226212046,0.08316353168627089,0.1004144232498759,0.1097758618258776,0.1104558655748635
6.7,0.005407370782485125,0.01754217705042251,0.03622938424265033,0.05876629311891542,0.08091229147525246,0.09843354097418335,0.1084224705768383,0.1099174540878381
6.800000000000001,0.005105689160609535,0.01668663498016303,0.03471868629214483,0.05673455893255431,0.07869568892886164,0.09644875018534235,0.1070261369432519,0.109308583543388
6.9,0.004821359971245364,0.01587281818903397,0.033267383801593,0.05476122275216719,0.07651498274366392,0.09446310924748844,0.1055906761862563,0.1086325756346117
7,0.004553342921640174,0.01509869171115925,0.0318734004514812,0.05284542098905737,0.07437126772012283,0.09247948673085041,0.1041197748081719,0.1078927345193254
7.100000000000001,0.004300663674745038,0.01436231982711971,0.03053471209068977,0.05098623538627498,0.07226548528129911,0.09050056781063809,0.1026169890994448,0.1070923385759218
7.2,0.004062409346773289,0.01366186122364628,0.02924934729676768,0.04918270040512661,0.07019843351224242,0.0885288607292279,0.1010857442576291,0.1062346328750735
7.300000000000001,0.00383772434215234,0.01299556438937767,0.02801538769771208,0.0474338100212285,0.06817077673109938,0.086566703288742,0.09952933402740517,0.1053228223346361
7.4,0.003625806497653241,0.01236176323516969,0.02683096808263397,0.04573852397012787,0.06618305460383049,0.08461626934473659,0.09795092081366917,0.1043600655251752
7.5,0.003425903510139483,0.01175887292800455,0.02569427632604612,0.04409577348001709,0.06423569081511527,0.08267957527503202,0.09635353622267297,0.10334946909379
7.600000000000001,0.003237309624752144,0.0111853859280828,0.02460355314811629,0.04250446652671462,0.0623290013085613,0.08075848640075782,0.09474008198901322,0.1022940827742932
7.7,0.003059362562475526,0.01063986821918858,0.02355709173106154,0.04096349264387606,0.06046320210972462,0.0788547233394614,0.09311333124897592,0.1011968949523088
7.800000000000001,0.002891440667935384,0.01012095572290219,0.022553237209896,0.03947172731931856,0.05863841674572961,0.0769698682726712,0.09147593012333818,0.1000608287544726
7.9,0.002732960259995687,0.00962735088769346,0.02159038605396593,0.03802803600638919,0.05685468327544363,0.07510537111261865,0.08983039957520095,0.09888873863161463
8,0.002583373169261507,0.009157819444367089,0.02066698535409205,0.03663127777746836,0.05511196094424543,0.0732625555549367,0.08817913751079275,0.09768340740658221
\end{filecontents*}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}
\usetikzlibrary{intersections,calc}
\begin{document}
\begin{tikzpicture}
\begin{axis}[%
no markers,
height=5cm, width=10cm,
smooth,
axis x line=bottom, axis y line=left,
xtick=\empty, ytick=\empty,
clip=false,
enlargelimits=upper,
restrict x to domain = 0:7,
restrict y to domain = 0:0.8,
]
\addplot[name path=plot1] table[x = x, y = pdf1]  \dataChiSquare;
\path[name path=y0] (0,0) -- (4,0);
\path[name path=p1] (1.2,0) -- (1.2,0.8);
\coordinate[overlay] (X) at (1,1);
\end{axis}
\path [name intersections={of=plot1 and p1, by=Q}];
\path let \p1=(X),\p2=(Q) in \pgfextra{\pgfmathsetmacro{\yQ}{\y2/\y1}
\typeout{y\space coordinate\space of\space Q\space is\space \yQ}};
\draw [name intersections={of=y0 and p1, by=P}]   (P) -- (Q);
\end{tikzpicture}
\end{document}


• Thank you. It works but I can't understand the code, could you please explain me how does the \path let ... \typeout ...; part work? Jun 19, 2018 at 1:10
• @Taekwondavide Sure. I put a coordinate (X) at (1,1) such that I know how much pgfplots stretches the coordinates. Then I compute the absolute coordinates for (X) and the intersection point (Q), and divide the absolute y coordinate of (Q) by the one of (X) such that I know the y coordinate in pgfplots units. In principle, one should be able to do that with \pgfplotspointgetcoordinates but there are the usual expansion issues that make this too cumbersome (for me).
– user121799
Jun 19, 2018 at 1:22
• Thank you! I managed to do that with a few instruction less, starting from your code, by putting this after axis environment: \path[name path=Qv] (1.2,0) -- (1.2,0.8); \coordinate[overlay] (X) at (1,1); \path [name intersections={of=plot1 and Qv, by=Q}]; \node[coordinate] (Q0) at (axis cs: 1.2,0) {}; \draw (Q0) -- (Q); Jun 22, 2018 at 15:43
• @Taekwondavide Sure. I thought you wanted the value \yQ`. Anyway, if you are happy, everything is good. ;-)
– user121799
Jun 22, 2018 at 15:45
• Whops, sorry, my fault... I wrote that I needed the coordinates but I ment the node :-( However you provided me everything I need! Jun 22, 2018 at 15:51