Document raster direction in plot_image.py.

diff --git a/src/plot_image/plot_image.py b/src/plot_image/plot_image.py
index 0262cfdad5fe2ca666d240bf843ca78cb00ca98a..4854eeadfd9aef2ce52a095ce18f36d1fb323fe9 100755 (executable)
--- a/src/plot_image/plot_image.py
+++ b/src/plot_image/plot_image.py
@@ -10,7 +10,20 @@ Step 2: pipe data into python script::
 
     ./gen_data | ./plot_image.py -s nx,ny -c nc -t 'image title'
 
-Data should be one ASCII float per line,
+Data should be one ASCII float per line, in the following order::
+
+    z[x1,y1]
+    z[x2,y1]
+    ...
+    z[nN,y1]
+    z[x1,y2]
+    ...
+    z[xN,y2]
+    ...
+    z[xN,yN]
+
+where `x` increases from `x1` to `xN` and `y` increases from `y1`
+through `yN`.
 
 For  usage info, run::