Question

For example, what is a code for plotting y = x so that y- and x-axis have the same scale ?

Answer 1

Maxima, by default, uses gnuplot to produce 2d graphics.

If you’re using this default, then you need to set the gnuplot_preamble, e.g.

plot2d(x, [x,-1,1], [gnuplot_preamble, "set size ratio -1"])$

Note the -1 means that gnuplot tries to set the scales so that the unit has the same length on both axes, while +1 sets the aspect ratio ignoring the axis scales.
To make this the default, type

set_plot_option ([gnuplot_preamble, "set size ratio -1"])$

If you want to find out more about the gnuplot size options, run gnuplot from your terminal and type help set size into the console.

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n.b. Most basic questions you can have about Maxima can be found somewhere in their mailing list archive: http://maxima.sourceforge.net/maximalist.html