#3772 plot2d does bad job on clipped lines

[Stavros Macrakis]

Plot2d doesn't do a good job on clipped lines, even of modest slope.
In the plot below, each clipped line begins and ends at a different place short of the edge of the plot.

plot2d(x-floor(x),[x,0,5],[y,.2,.4]);

Clipping short of min/max y is disconcerting, and increasing adapt_depth doesn't help (though increasing nticks does).

Maxima 5.44.0 SBCL 2.0.0 Windows 10 gnuplot 5.2/PL8