plotting - Hatching pattern in ColorRules for MatrixPlot

Using ColorRules with MatrixPlot, how does one get a hatching pattern in one of the cells?

For example, consider the data,

data = { {1, 2, 1}, {2, 2, 3} }

And now I consider the MatrixPlot,

MatrixPlot[data,
 ColorRules -> { 
   1 -> Blue, 
   2 -> Red, 
   3 -> Purple 
   }
 ]

Question: For the cell with the value 3 and currently with the color Purple, how can I get it to have a Purple color with a hatched pattern?

Answer

As belisarius said, there is no current support for hatching but you could use a graphics overlay as a workaround.

First create the desired pattern and texture a Polygon with it:

t = Table[{0, n}, {n, -1, 1, 0.1}];

g[c_] := Graphics[{AbsoluteThickness[8], Line /@ Transpose[{t, t + 1}]}, 
    PlotRange -> {{0, 1}, {0, 1}}, Background -> c];


pattern2[p_, c_] := Graphics[{Texture[g[c]], Polygon[{
     {p[[1]] - 1, p[[2]] - 1},
     {p[[1]], p[[2]] - 1},
     {p[[1]], p[[2]]},
     {p[[1]] - 1, p[[2]]}},
    VertexTextureCoordinates -> {{0, 0}, {0, 1}, {1, 1}, {1, 0}}
    ]}]

pattern2[{1, 1}, Purple]

Create a function to place the pattern over any field of a given value n. matrixLength is the length of the input data, c the color.

overlay[patternFunc_, n_, c_, matrixLength_] := 
  Show[patternFunc[#,c] & /@ ({#2,matrixLength + 
     1 - #1} & @@@ Position[data, n])];

Example:

plot = MatrixPlot[data, ColorRules -> {1 -> Red, 2 -> Orange, 3 -> Purple}]

Show[
 plot,
 overlay[pattern2, 3, Purple, Length@data]
 ]

Not the most elegant/efficient solution but it might be useful as a starting point.