How to plot reference lines of a slope in matlab?

I am solving a ode numerically I also plot discrete L2 norm in order to check the convergence of my numerical scheme now i want to plot reference line for slope 2,3 in my figure so that one can see the convergence of numerical solution to the exact one. is there some one to tell me the solution. I also attached my convergence plot here.

Thanks in advance.

[Stephen23]

@Bibigul: What exactly is a "reference line" ? Is it a line of best fit, or a line with a constant slope, or a grid line, or something else ?

[Bibigul]

@Stephen: a line with a constant slop. Actually i want to insert lines with constant slop of 2,3,4 and 5 in my plot.