http://blog.touringplans.com/2015/05/06/fastpass-plus-availability/
I just can’t understand how to read it. There are times, and there are colors. The description says the colors corresponds to the time the FastPass+ runs out for that attraction, based on crowd level predictions. But … so does the time listed, right?
So why are the times and the colors not matching up? For instance, sometimes 4pm is listed in light red, sometimes light blue. What does that mean?
The use of color is just a way to enhance results, for easy visualization of whether it’s a day of scarce FPP for a given attraction (red shades) or plentiful FPP (blue shades). It’s easy to see which ones are going to be tough to get, at a glance. I’m a visual learner - I like the added benefit of the color.
4pm is in the middle of the times, and is colored in the middle (sometimes white, sometimes lightest red or lightest blue). I can’t speak to any lack of consistency about that, but at least the 4pm spots are always the middle color. Each park has a “key” below its chart showing the color range from dark red to dark blue corresponding with earliest times to latest times. I didn’t see any color issues with other times.
The colors are relative to the park and how fast the Fastpasses run out for that park. So the times of day will be different colors for each park.
For example, 4 p.m. at the Magic Kingdom indicates a relatively high demand for Fastpasses, so it’s reddish. 4 p.m. is neutral at the Studios, so it’s white.
I was hoping this thread would make me finally understand that chart… but no - I’m still very confused 
Example: Why do FP+ run out for the Jungle Cruise at 5 PM with CL 1, but not until 10 PM with CL 9?
See the link above. Look below the graphs at the paragraph that begins with “A reasonable assumption…” for your answer.
I have often wondered that myself. I try to remind myself that the chart merely reflects the data. Also, the differences in availability on the various crowd levels may be a reflection of the way the crowds move and behave differently on those days.
Maybe Len has more insight, but this is what my gut tells me
My guess… and it’s JUST a guess… lower crowds = fewer FPP made available by WDW. It wouldn’t do to have everyone have a FPP and no lines in the standby area.
But, I could be completely and utterly wrong. 
What @Sam2071 says is correct. Disney manipulates Fastpass availability based on crowd levels, sometimes making more available when crowds are larger. (That is, they allocate more of the ride’s capacity to Fastpass, when crowds are larger.) That comes, of course, at the expense of the people in the standby line.
I seem to remember from the original article that park closing times also tend to be earlier on lighter crowd days, making later-in-the-day FP+ availability more scarce accordingly. Can’t get an 8PM Tower of Terror FP+ if DHS is closing at 7PM, after all.
Thanks, all of you, I finally got it! 
I will take a closer look at the chart now that I finally understand it. We will be in the parks in early October, so maybe FP+ isn’t necessary for the non-headliners in MK, but I am very happy to understand the chart anyway!