Add Normal Curve to Histogram

[dpierucci]

It would be nice for Brunel to have support for a normal curve in Histograms.

[grahamwills]

One of the biggest challenges with adding a normal curve is matching the areas under the curves -- the histogram can have a very different height, depending the bar widths. So would the following be acceptable:

The normal curve would be on a fixed scale (probably 0-1) and would be drawn as a second element using the "dual y axis" feature, hiding the Y axis. So the areas under the curve would not match up, with the normal curve height staying the same regardless of the degree of binning. It would also mean that the chart could not be a dual-Y axis chart, as that feature would be "used up" by the normal curve