When there is 10+ series in a Scatter plot for examples - Legend look really bad.. It just extends to bottom of the screen - crossing chart area etc.
What it would be nice is to have a drop-down list that allows multiple selections : http://www.w3schools.com/tags/tryit.asp?filename=tryhtml_select_multiple
And color / legend next to each selection outside of the chart (not inside the chart). That way it would be elegant solution for many series where user can scroll down and select multiple series at once that he/she wants to see (not clicking only 1 at the time)..