FR: return slope coefficient and R2 statistic for LinearAnalysis

[floswald]

Problem description

I often want to show a scatter plot with a regression line, next to which I want to print the slope coefficient of the line (or maybe the entire equation like in this Plots.jl solution). Of course I imagine one can use annotation also in the Makie plots, but it would be nice if the placement of the equation would be automated.

Proposed solution

• add a kw equation = false to linear. if equation = true, return beta from y = a + beta x + e.
• the tricky part is to find a suitable location for the annotation on the scatter plot.