Support trendline annotations

[mdfederici]

I would like to be able to leverage annotations to specify a trendline like so:

I can do this with the current annotation code leveraging 'ax' and 'ay' but it is a bit clunky (as i need to convert a point on the graph to pixels outside of plotly) and does not work correctly when the plot is zoomed.

How I imagine this working is a flag in the options which when set to true tells the annotation code to interpret 'ax' and 'ay' not as an offset in pixels but rather an absolute point in the coordinate system which indicates the position of the tail (while x,y continues to represent the head as usual). This would make it easier to draw the trendline and the trendline would correctly scale with zoom like so:

I've never contributed to plot.ly before but I've implemented this in my plotly fork and have a pull request there (in accordance with the contributing guidelines).

[mdfederici]

pull request to plotly/plotly.js

[mdfederici]

@etpinard @mdtusz any chance we can get this one in for this week's release to npm? thanks guys
MFed

[etpinard]

@mdfederici I'm a little caught up with a big project. I'll review your PR either tomorrow or Friday. Thanks for your contribution!

[etpinard]

@mdfederici My bad. I have to wrap up an important PR today. I'll review yours on Monday. Sorry for the wait.

[mdfederici]

@etpinard no worries. finishing important things is good :)

[etpinard]

done in #610