Capn Bloggs;
The chart is not for a particular altitude. It covers all weights and altitudes.
"delta" stands for the ambient pressure at altitude expressed as a ratio to sealevel standard pressure, i.e delta=Pamb/Psls.
P.S.
The more I look at it, the less that chart makes sense.
If you plotted a family of curves on the graph, each curve showing for a particular weight and altitude the variation of L/D versus Mach, then each curve would have a maximum L/D at a particular Mach. The L/D curve shown on my chart connects all these maxima, i.e. all curves for any weight or altitude would stay below the L/D curve on the chart.