When using int charttype = SPCControlChartData.MEAN_SIGMA_CHART_VSS; this.PrimaryChart.Datatooltip.EnableCategoryValues = true;
and looking at the resulting tool tips I am not able to see the left edge of the tool tip window. Also, the values don't appear to be formatted correctly.
I have looked at the sample charts provided for SPCControlChartData.MEAN_SIGMA_CHART_VSS and they have this.PrimaryChart.Datatooltip.EnableCategoryValues set to false. When I set it to true in the sample chart then the problem exists there as well (unless the number of samples is equal to numsamplespersubgroup).
What can I do to make this work regardless of sample size?
Yes, that looks like a bug. The tooltip is trying to process uninitialized values, resulting in a formatting error. We will have to investigate, though it is unlikely there is any way to resolve the problem with the current release of the software, other than not to enable the display of those values in the data tooltip. We will address the problem in an intermin release in a couple of weeks.