Line Chart: Add feature to be able to remove data (and labels)

[powertec-dan]

Currently the only way to update a chart to remove data seems to be to use the Update method.

While this works, it forces the chart to redraw in full.

A desirable feature would be to have a function call that allows the removal of one or more items of data and their corresponding labels.

This would enable the use-case where there is a live graph displayed and data (and labels) being added to the right-hand side of the graph while old data (and labels) is removed from the left hand side.