Adaptive gridlines

Question

I want to use gridlines to create an effect of millimeter graphing paper on a 2d graph, to show how multi-variable function depends on 1 variable. The scales of different va

Answer 1

I think FindDivisions[ ] is what you're after:

FindDivisions[{xmin,xmax},n] finds a list of about n "nice" numbers that divide the interval around xmin to xmax into equally spaced parts.

getTicks[x_, y_] := Flatten@FindDivisions[#, {10}] & /@ FromTo[x, y]
getGrid  [x_,y_] := FindDivisions[{x,y},{10,5}]/.
                          {r__,{s__}}:>Join@@{s,{#,{Gray,Thick}}&/@r} 

Answer 2

If you use the same function for FrameTicks and Gridlines, they'll line up.

See FrameTicks, and GridLines. I think you'll need ImageMargins for the border.

Answer 3

Don't use FrameTicks but shift the grid correctly. This is a first approach. Dinner waits.

getGrid[min_, max_] :=
 Module[{step, i},
  Print[{min, max}];
  step = 1/100;
  Table[
   {
    Floor[min, 0.1] + i*step,
    If[Equal[Mod[i, 10], 0], Directive[Gray, Thick, Opacity[0.5]],
     If[Equal[Mod[i, 5], 0], Directive[Gray, Opacity[0.5]],
      Directive[LightGray, Opacity[0.5]]
      ]
     ]
    },
   {i, 1, (Ceiling[max, 0.1] - Floor[min, 0.1])/step // Round}
   ]
  ]

Use an AspectRatio that's appropriate for the grid (probably the ratio of x and y ranges)

---

After-dinner update

To make it more robust for different value ranges (per your comment) I generate the ticks that would be chosen by ListPlot and base my steps on that:

getGrid[min_, max_] :=
 Module[{step, i,j},
  i = Cases[(Ticks /. 
       AbsoluteOptions[ListPlot[{{min, min}, {max, max}}], 
        Ticks])[[1]], {a_, ___, {_, AbsoluteThickness[0.25`]}} :> a];
  step = i[[2]] - i[[1]];
  Table[
   {
    i[[1]] + j*step/10,
    If[Equal[Mod[j, 10], 0], Directive[Gray, Thick, Opacity[0.5]],
     If[Equal[Mod[j, 5], 0], Directive[Gray, Opacity[0.5]],
      Directive[LightGray, Opacity[0.5]]
      ]
     ]
    },
   {j, 0, 10 Length[i]}
   ]
  ]

and getting the aspect ratio which yields a square raster

getAspect[{{minX_, maxX_}, {minY_, maxY_}}] :=
 Module[{stepx, stepy, i, rx, ry},
   i = (Ticks /.AbsoluteOptions[ListPlot[{{minX, minY}, {maxX, maxY}}], Ticks]);
   rx = Cases[i[[1]], {a_, ___, {_, AbsoluteThickness[0.25`]}} :> a];
   stepx = rx[[2]] - rx[[1]];
   ry = Cases[i[[2]], {a_, ___, {_, AbsoluteThickness[0.25`]}} :> a];
   stepy = ry[[2]] - ry[[1]];
  ((maxY - minY)/stepy)/((maxX - minX)/stepx)
  ]

---

Test

ELP[x_, y_, ex_, ey_, name_] := 
 ErrorListPlot[Cmb[x, y, ex, ey], PlotLabel -> name, Joined -> True, 
  Frame -> True, GridLines -> getGrid, ImageSize -> {600}, 
  PlotRangePadding -> 0, AspectRatio -> getAspect[FromTo[x, y]], 
  PlotRange -> FromTo[x, y]]


ELP[{4124961/25000000, 27573001/100000000, 9162729/25000000, 
  44635761/100000000, 15737089/25000000, 829921/1562500, 
  4405801/4000000, 23068809/25000000, 329386201/100000000, 
  58079641/100000000}, {1/10, 1/5, 3/10, 2/5, 3/5, 1/2, 1/2, 1/2, 1/2,
   1/2}, {2031/(250000 Sqrt[10]), 5251/(500000 Sqrt[10]), 
  3027/(250000 Sqrt[10]), 1/100000 6681/(500000 Sqrt[10]), 
  3967/(250000 Sqrt[10]), 911/(62500 Sqrt[10]), 
  2099/(100000 Sqrt[10]), 4803/(250000 Sqrt[10]), 
  18149/(500000 Sqrt[10]), 7621/(500000 Sqrt[10])}, {1/2000, 1/1000, 
  3/2000, 1/500, 3/1000, 1/400, 1/400, 1/400, 1/400, 1/400}, "T2, m"]

Here I divide the y-values by 20 and multiplied the x-values by 10000 to show the grid is still good:

---

Final update (I hope)

This uses FindDivisions as suggested by belisarius. However, I used the three level line structure standard for milimeter paper as requested by Margus:

getGrid[x_, y_] := 
 FindDivisions[{x, y}, {10, 2, 5}] /. {r_, s_, t_} :> 
   Join[
     {#, Directive[Gray, Thick, Opacity[0.5]]} & /@ r, 
     {#, Directive[Gray, Opacity[0.5]]} & /@ Union[Flatten[s]], 
     {#, Directive[LightGray, Opacity[0.5]]} & /@ Union[Flatten[t]]
   ]

and

getAspect[{{minX_, maxX_}, {minY_, maxY_}}] :=
 Module[{stepx, stepy},
  stepx = (#[[2]] - #[[1]]) &@FindDivisions[{minX, maxX}, 10];
  stepy = (#[[2]] - #[[1]]) &@FindDivisions[{minY, maxY}, 10];
 ((maxY - minY)/stepy)/((maxX - minX)/stepx)
  ]

---

WARNING!!!

I just noticed that if you have this in MMA:

and you copy it to SO (just ctrl-c ctrl-v), you get this:

(maxY - minY)/stepy/(maxX - minX)/stepx  

which is not mathematically equivalent. It should be this:

((maxY - minY)*stepx)/((maxX - minX)*stepy)

I corrected this in the code above, but it has been posted wrong for half a day while working correctly on my computer. Thought that it would be good to mention this.