The bigger the chart/differences in the chart get the harder it get's to point the cursor to display a label. Also if you have multiple y-axis the label is only shown for the one your touching - it would be nice if in discrete charts the label would show the values of all axis.

tl;dr;
- label should show for closest data point for easier touching
- label should contain all values

that's e.g. how it works with recharts: http://recharts.org/en-US/examples/SimpleLineChart