Refactor standard equations challenge
Refactor y-intercept challenge
Fix robots controlling wrong controls
Use slightly more meaningful fn name
Remove 'x + -3' from explore
Factor chart display, and add soln span
Move scripts to head
Add a solution-point marker to chart
Let soln be anywhere, and plot it first

It looks better if the lines get printed over it, rather than vice versa.
Hide solution ordered pair for now