pybootchart/draw: Avoid divide by zero error

When disk stats don't run frequenctly enough, we see divide by zero
errors. The code already has a fallback path so ensure we use it
for this case too.

[YOCTO #14360]

(From OE-Core rev: b71d30aef5dc2c360432c0dd4147859dd303ea48)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/pybootchartgui/pybootchartgui/draw.py

@@ -271,7 +271,7 @@ def draw_chart(ctx, color, fill, chart_bounds, data, proc_tree, data_range):
     # If data_range is given, scale the chart so that the value range in
     # data_range matches the chart bounds exactly.
     # Otherwise, scale so that the actual data matches the chart bounds.
-    if data_range:
+    if data_range and (data_range[1] - data_range[0]):
         yscale = float(chart_bounds[3]) / (data_range[1] - data_range[0])
         ybase = data_range[0]
     else: