plotting - How do I change axes scale?

I am plotting

f[x_] := 2*x + 1
Plot[f[x], {x, -10, 10}]

BUT I want to scale my axes differently. By default it is 1:2 , meaning 1 unit on $y$ axis is equal to 2 units on $x$ axis. I want to manually specify it. I tried to use AspectRatio, but this only changes my graph, and not the axes... What can I do?

Answer

I could interpret this question a couple of different ways. One is that you would like to scale the proportion of your plot. One can use ScalingFunctions (though undocumented for Plot) to specify the intended relationship, which will be correctly handled with AspectRatio -> Automatic. For example:

Table[
 Plot[3 Sin[2 x], {x, 0, 7}, ScalingFunctions -> {scale, Identity}, 
  AspectRatio -> Automatic, PlotRange -> All],
 {scale, {{2 # &, #/2 &}, Identity, {#/2 &, 2 # &}}}

] // GraphicsRow

Note that unlike setting the proportion using a specific value for AspectRatio I did not have to manually calculate this value; it was computed automatically.

Another interpretation is that you wish to relabel one of the plot axes, easily done with a coefficient in the right place:

GraphicsRow[{
  Plot[3 Sin[2 x], {x, 0, 7}],
  Plot[(3 Sin[2 x])/2, {x, 0, 7}],
  Plot[3 Sin[2 (x/2)], {x, 0, 14}]
}]

These two behaviors may be combined to produce most any scaling you desire.