They describe the graph as such:
"...(Graph of relative flexibility and cost of power plant technology. Flexibility is on the x-axis and costs is on the y-axis. Both low: nuclear and coal. Both Medium: combined-cycle gas turbines and coal. Both high: simple-cycle turbines. Low cost and high flexibility: hydroelectric)
The minimum run time and ramp times determine how flexible the generation source is; these vary greatly among types of plants and are a function of regulations, type of fuel, and technology. Generally speaking, plants that are less flexible (longer minimum run times and slower ramp times) serve base load energy, while plants that are more flexible (shorter minimum run times and quicker ramp times) are better-suited to filling peak demand. Table 5.2 and Figure 5.3 show approximate (order-of-magnitude) minimum run times and ramp times for several generation technologies. It is important to realize that, in some sense, these are "soft" constraints. It is possible, for example, to run a nuclear plant for five hours and then shut it down. Doing this, however, imposes a large cost in the form of wear and tear on the plant's components.
The cost structure for transmission and distribution is different than for power generation, since there is basically no fuel cost involved with operating transmission and distribution wires (and their associated balance-of-systems, like substations). At the margin, the cost of loading a given transmission line with additional electricity is basically zero (unless the line is operating at its rated capacity limit). Capital cost thus dominates the economics of transmission and distribution..."
For my part, I fully appreciate the rigors of modern life and use of power, so I am more amenable to power sources that don't fluctuate due to lack of wind and sunlight, but I have no problem with those kinds of energy sources (especially if I don't have to have my tax dollars used to subsidize them to the degree they currently are)...I also would have less of an issue with windmills and acres of solar panels if they could deliver all the time on demand...I could live with that.
Perhaps someday there will be a good way to store energy from those types of sources to be used during those slack times they are prone to.