Improve graphing function
Add a label to graph (not coloured)
Fix answer
Improve question formating
Improve question formating further and allow coefficients to equal 1
Colour legend
Use randRangeNonZero
Remove colour from question
Make one line dashed
Merge pull request #43304 from petercollingridge/graph-legend

Refactor code, add f(x)/g(x) labels to the graph, and make one of the lines dashed in Functions 2.

Fixes #43277, #43257, #42625, #43273, #42172, #42025, #42038, #41921, #38788, #38960, #43288, #43309