Your graph goes from 270 to 350 parts per million, a delta of 80 units per about 5 inches of vertical computer screen space. If you were to graph the total atmosphere content at this same scale the graph size would have to stretch from your computer screen 1 mile high. If you were to shrink that full graph then the CO2 content would appear to be a straight line, though it wouldn't be large enough to activate even one rounded up pixel on your screen.
My suggestion is that you fire up Excel, graph 20-30 estimated data points based on the first version of that graph, over-ride the default scale to start the Y axis at 0, and then post your result.