axis(ax, 'tight') % where ax is your axis handle.
will set the axis limits so the edges of the plot touch the max and min of your data.
After running that command, you can run the lines below to add +/- 5% of your data range to each side of the x and y axes.
xlim(ax, xlim(ax) + [-1,1]*range(xlim(ax)).* 0.05)
ylim(ax, ylim(ax) + [-1,1]*range(ylim(ax)).* 0.05)
If you only want to apply this to the x-axis limits,
yl = ylim(ax);
axis(ax, 'tight')
ylim(ax, yl)
xlim(ax, xlim(ax) + [-1,1]*range(xlim(ax)).* 0.05)
