Is there a visual way to represent drag capability?

In this data visualization chart, we wanted to make user understand that the chart is interactive.

When someone clicks and drags any point, the selection expands. This helps drilling down in the data. However when we see the chart, we don't understand that it's interactive.

We can show a label like:

Drag the points to expand selection

We can also train user - this will be expensive.

However, is there a visual way to represent drag capability?