It would be very useful to be able to add trendlines (linear, power etc) to charts (particularly scatter graphs) as in Excel.
Whilst not quite as efficient, the ability to overlay a line graph onto a scatter would enable the user to achieve a similar result as the points for the line can be calculated.
@daniel_mmi - I will take a closer look at this idea submission as it is an older one and I do not think the status was reported properly, but in the meantime, does the Scatterplot tool give you what you need? If you install the Predictive tools, the Scatterplot tool is part of the Data Investigation group. There is also a sample that uses the Scatterplot tool: Help ->Sample Workflows -> Predictive Analytics -> 4 scatterplot.yxmd