flip a gray scale image data

Hello,

I am using Gnuplot 4.2.2 to plot an image from an ASCII data file. The
file has the data in the following format:
x1 y1 V1
x2 y2 V2
..
..
..

where x and y give the coordinates (columns and rows) and V's give the
gray value.


I am getting an image using this command:
plot 'image_data.pgm'  with image


but I am getting an upside down image.


How do I flip the image and invert the y-axis? It appears that flipy
only works for binary data (http://www.gnuplot.info/docs/node115.html).

Thanks.