Sim plot: fix a rounding issue that can create a missing point in a plot.

Due to a truncation when converting a value to integer, the first point was
sometimes missing because it looked like out of plot range.
Fix also a cosmetic issue in dialog.
Fixes #7345
https://gitlab.com/kicad/code/kicad/issues/7345

common/widgets/mathplot.cpp

@@ -708,7 +708,9 @@ void mpFXY::Plot( wxDC& dc, mpWindow& w )
                 wxCoord y1 = w.y2p( py );
 
                 // Store only points on the drawing area, to speed up the drawing time
-                if( x1 >= startPx && x1 <= endPx )
+                // Note: x1 is a value truncated from px by w.x2p(). So to be sure the
+                // first point is drawn, the x1 low limit is startPx-1 in plot coordinates
+                if( x1 >= startPx-1 && x1 <= endPx )
                 {
                     if( !count || line_start.x != x1 )
                     {