Display correct equation with multiple trendlines in chart.

When there were multiple trendlines for one data series and one
was selected, always the equation and R^2 of the first trendline
was displayed in status bar.

Change-Id: I320261e129c51bbdd1228173ca2f1d8447fdcb89
More clearly name the trendline in chart's element selection box.

Change-Id: I5f787eb064524a3b2399d591866a5b8cbdee6294