Why does PGFplots not recognize my expression?

Question

I want to graph two quadratic equations using pgfplots. The first one work fine, however when I add the second equation, I get the following error message:

Package PGF Math Error: Unknown operator `x' or `x+' (in 'x^2 - 3x + 4').

It seems PGFplots will not let me multiply numbers at all. Functions such as 2x or even x * 2 don't seem to work. Here is what I am working with:

\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usepackage{pgfplots}
\begin{tikzpicture}
\begin{axis}[
    axis lines = center,
    xlabel = {$x$},
    ylabel = {$ y = f(x)$}
]
    \addplot[no markers, red]{x^2};
    \addplot[no markers, blue]{x^2 - 3x + 4};
\end{axis}
\end{tikzpicture}

The red plot works fine, but the blue one causes a compilation error.

Answer 1

pgfplots doesn't assume multiplication between scalars and variables, so your expression should be x^2 - 3*x + 4:

\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
    axis lines = center,
    xlabel = {$x$},
    ylabel = {$y = f(x)$}
]
    \addplot[no markers, red]{x^2};
    \addplot[no markers, blue]{x^2-3*x+4};
\end{axis}
\end{tikzpicture}
\end{document}

Answer 2

Try:

 \addplot[no markers, blue]{x^2-3*x+4};

That should work.