## Regression lines need labeling features

##
`@lukehutch`

Submitted by Luke Hutchison **Link to original bug (#605041)**

## Description

Regression lines need a number of other features to make them really useful:

(1) When adding a regression line to a graph, it is often really important to display the "r value" (Pearson's correlation coefficient, given by =correl() ) right next to the regression line. This should be manually moveable in case the automatically-assigned location is undesirable.

(2) The actual equation of the regression line (whether linear or otherwise) needs to be able to be added (and possibly repositioned) too.

(3) It would be immensely helpful if "axis intercept lines" could be added to any regression line: given a certain X value, calculate the corresponding Y value of the regression line for that X value, and then draw a vertical line from the X axis up to the regression line, and then a horizontal line from the regression line to the Y axis. These lines could optionally extend the entire width and height of the graph, or just the partial distance from the axis to the regression line. This allows for certain points of interest (e.g. thresholds) to be clearly shown on the graph.

Version: GIT