scripts: plot.py: Fixed subplots with differing widths

This was completely broken due to the renderer ignoring s.xspan. As a
result, subplots could end up rendered multiple times if they spanned
neighboring subplots.

Quick example:

  ./scripts/plot.py test.csv -xx \
      --subplot="-ya" \
      --subplot-right="-yb" \
      --subplot-below="-yc"

Fortunately the fix is easy, just make sure to increment x_ += s.xspan
as we render subplots across the x-axis.

Curiously the behavior was already correct for the y-axis, I guess
because the y-axis is quite a bit more complicated with how it crosses
multiple lines.

scripts/plot.py

@@ -1811,7 +1811,8 @@ def main_(ring, csv_paths, *,
                         for line in ylabel_)
                 if ylabel_ else '')
 
-        for x_ in range(grid.width):
+        x_ = 0
+        while x_ < grid.width:
             # figure out the grid x/y position
             subrow = row
             y_ = len(heights)-1
@@ -1821,6 +1822,7 @@ def main_(ring, csv_paths, *,
 
             s = grid[x_, y_]
             subrow = row - sum(heights[s.y+s.yspan:])
+            x_ += s.xspan
 
             # header
             if subrow < s.ymargin[-1]: