Stop gnuplot interpolating heatmaps – WhereIsMyAnswer

I am trying to plot some data as a heat map in gnuplot. My data is a “.txt” file with three columns. The first two columns are the independent variables and the third is the output.
E.g, a simplified example:

0.00 0.00 11.9
0.00 0.50 0.01
0.50 0.00 4.5
0.50 0.50 20.0

gnuplot> set view map; set dgrid3d; splot "output.txt" using 1:2:3 with pm3d

I expect to get a 2*2 array of squares with colours according to their value. Instead, I think gnuplot has interpolated the values as regardless of my initial number of data points, it plots a 9*9 array of squares with shading. How can I just plot the data I have with no interpolation?
output from gnuplot