Plot function in TikZ from an array of functions - tikz

Question

I am trying to create a code that automatically draws splines (parametric curves of arbitrary degree—not only cubic—defined as piecewise functions of the variable \t) in 2D. I will compute those functions within an .Rnw file and I’d want my TikZ code to plot the different pieces within a for loop. My problem is that I don’t know how to create (and access) an array of such functions.
I provide a very simplified MWE here
\documentclass{article}
\usepackage{tikz}
\usepackage{ifthen}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
\def\npcs{2}
% Functions defined separately
% 1st section
\def\fxa{-2* \t^0 + 2* \t^1 + 0* \t^2}
\def\fya{2* \t^0 + 2* \t^1 -1* \t^2}
% 2nd section
\def\fxb{-2* \t^0 + 2* \t^1 + 0* \t^2}
\def\fyb{1* \t^0 + 4* \t^1 -2* \t^2}
% ... as many times as sections in the piecewise functions
% Functions defined with an array
\def\fxall{{-2* \t^0 + 2* \t^1 + 0* \t^2}, {-2* \t^0 + 2* \t^1 + 0* \t^2}} % ... as many elements as sections in the piecewise functions
\def\fyall{{2* \t^0 + 2* \t^1 -1* \t^2}, {1* \t^0 + 4* \t^1 -2* \t^2}} % ... as many elements as sections in the piecewise functions
\draw[thick, domain = 0:1, smooth, variable = \t, color = red!60]
plot (\fxa, \fya);
\draw[thick, domain = 1:2, smooth, variable = \t, color = red!20]
plot (\fxb, \fyb);
% This is the part that is not working!!
\foreach \n [evaluate = \n as \tstart using int(\n - 1), evaluate = \n as \color using int(100 - 40*\n)] in {1,...,\npcs}
{
\draw[thick, domain = \tstart:\n, smooth, variable = \t, color = red!\color]
plot (\fxall[\tstart], \fyall[\tstart]);
}
\end{tikzpicture}
\end{document}
If I define the functions by hand: \fxa, \fxb, etc, \fya, \fyb, etc… then I would have no problem in plotting all of them, but that obviously does not automatize the problem.
However, when I try to automatize the plotting with a \foreach loop, I get an error. I have tried different ways of declaring the variables \fxall and \fyall (with double braces, with parentheses…) but nothing worked. I get a cascade of massage errors like
Missing $ inserted. }
Extra }, or forgotten $. }
Any ideas on how to fix this?
Thanks in advance!!