Feldspar content of coarse-grained igneous rocks
You can't really generalize like that for all igneous rock ranges.  The chart actually tells you what the type and (approximate) mineral composition of a rock are as functions of the chemical composition, specifically silica content.  You should be looking at how the minerals are distributed as a function of silica content.
To understand this diagram fully, note the following:

The horizontal axis is actually a continuous variable, typically the silica content you would obtain if you were to separate all the individual oxide components.  This diagram seems to go roughly from 50% to 70% from left to right; the one quoted here which includes rocks found predominantly in the mantle goes down to 40%.

The rocks are classified according to silica content and crystal or grain size; thus granite and rhyolite are similar in silica content and are regarded in the diagram as having similar mineral compositions; only the grain size is really different.


Thus the curved lines define how much of each mineral is typically in a rock having some level of silica content, which varies across the chart as indicated in point (1).  When we have a high silica content more than about 65% (felsic rocks) in a granite or rhyolite, there can be more alkali feldspar than plagioclase feldspar (the alkali feldspar composition range is higher in silica).  In contrast, a low-silica (mafic) rock won't have any alkali feldspar and even the more flexible plagioclase feldspar is limited; mafic rocks are higher in other minerals such as pyroxenes.