Attached is a small VI that generates 4 sinewaves and plots them in two equally configured WfmCharts.
ChartUpdateProblem.vi
The WfmCharts is configured as
Scope update mode
All plots have thicker lines to make the problem more visible
Plot1 and Plot 2 are anti-aliased
In this case the Stop button overlays the plot area in the upper WfmChart.
Run the VI and notice the difference in the lines in the upper and lower WfmCharts. The upper chart should have somewhat jagged lines, and if you run the VI for a while you might even get to see this ;-)
Pressing the stop button forces LabVIEW to redraw the upper chart.
If you move the stop button outside the plot area, the problem dissapears, but the same problem can be seen if you move the plot legend inside the plot area.
/J