Adding line parallel to quantile plot reference line

Question

I want to compare two list in a quantile plot

QuantilePlot[{1, 2, 3, 5, 8}, {2, 3, 6, 9, 10}]

I now want to add a black line parallel to the reference line, like this:

Show[QuantilePlot[{1, 2, 3, 5, 8}, {2, 3, 6, 9, 10}],
  Graphics[{Dashed, Line[{{2, 0}, {10, 6}}]}]]

but I how do I get the right intercept/slope for this line? I want to add a line above and below the reference line with a self-defined distance for visual aid.

Answer 1

qp = QuantilePlot[{1, 2, 3, 5, 8}, {2, 3, 6, 9, 10}];
li = Last@Cases[qp, _Line, Infinity];

Show[
 qp,
 Graphics[{
   Gray,
   li /. {a_Real, b_} :> {a, b + 1},
   li /. {a_Real, b_} :> {a, b - 1}}]]