If you're going to do this, you're better to do it in the luminosity tab of the spreadsheet. Piezography is linear in luminosity, not density. Then you'll get the black line and pink lines coinciding. All this chart does is demonstrate that Piezo is not linear in density, which it's not supposed to be.
[To explain a little more, the density tab of the linearisation checker spreadsheet is not what it appears to be. It's not actually plotting density. In some hidden columns, it converts the actual density numbers to luminosity and plots those. So the graph in the density tab is actually the same graph as the one in the luminosity tab, but gets there by converting the density numbers to luminosity first. I.e. the graph is not a plot of the actual vs ideal density numbers that you see in that tab. In that sense, it's a little misleading, because the ideal density numbers you see are not really "ideal". They should be the ideal luminosity numbers converted back to density, and then plotted, but this is a curved line, not a straight line.]