Right now there are two main areas that contribute to the whitespace there.
The first is the XYChart.PaddingPercentage property. Setting that to zero removes the padding that by default is present around plotted data. It defaults to 5%.
The second is the label positioning. In the case you show, the label positioner determines that near half of the label would normally extend past the axis if PaddingPercentage = 0. So it forces the resolved padding to be increased such that the label doesn't cross the axis. That is the main logic that causes the problem for your scenario.
What do you think the ideal solution for this is? Do all other chart libraries that you have used make sure the labels near the left/right always "slide in" instead of centering over the target data point? It seems like that might not be something people always want to do.
Any information you can provide here would be helpful.