More informative error message when data is too large for Agg to draw it.

svn path=/trunk/matplotlib/; revision=6172

Author: mdboom

agg24/include/agg_rasterizer_cells_aa.h

@@ -185,7 +185,10 @@ namespace agg
             if((m_num_cells & cell_block_mask) == 0)
             {
                 if(m_num_blocks >= cell_block_limit) {
-                    throw "Agg rendering complexity exceeded.";
+                    /* If this exception is thrown too often, one can
+                       increase cell_block_limit */
+                    throw "Agg rendering complexity exceeded. "
+                        "Consider downsampling or decimating your data.";
                 }
                 allocate_block();
             }