It’s an interesting idea but do keep in mind that there are cases where we can’t calculate these values.
Take a look at this example:

In this chart it wouldn’t be possible to resolve the years as each data point on the chart would be a different year.
Using this sample data, the legend would have to say something like this for each data point to be accurate:
2012 vs nothing
2013 vs 2012
2014 vs 2013
2015 vs 2014
Now, if i were to drill down into 2014 then it might make sense to change the legend to Revenue 2014 vs Revenue 2013. But then would everyone want this? I don’t know… It might be a bit odd for the legend to change behaviour as it drills down. Consistency is an important consideration.
If you want different behaviour, you’re probably best adding a script and changing the legend yourself so that it could be dynamic. To change the text programmatically, you can use the LegendText property and insert any custom behaviour that you like.
chart1.control.series[0].legendText = “my text here”;
If you feel really strongly about this, it’s certainly worth reaching out to our support team and opening a feature request.