You cannot draw this kind of plot without assigning numeric coordinate values to each of the X and Y labels. We as on-lookers have no reason to expect that NDJb is not, for example, immediately to the right of NDJa separated by 1/1234'th of the plot width. We as on-lookers would be justified in sorting the labels alphanumerically and using the sort order index as the X or Y coordinate in a scatter plot and do a gridded interpolation in order to fit find a surface to contour.
If you do assign equal-spaced numeric coordinates to the rows and columns in file order, say Xvalues (the columns) and Yvalues (the rows) and corresponding cell array of strings Xlabels and Ylabels (extracted from the file), then
contour(Xvalues, Yvalues, Z);
set(gca, 'XTick', Xvalues, 'XTickLabel', XLabels);
set(gca, 'YTick', Yvalues, 'YTickLabel', YLabels);
colorbar();
If your arbitrary Yvalues for the first data line of the file is numerically lower than your arbitrary Yvalues for the second data line of the file, then you should also add
set(gca, 'YDir', 'reverse')
