The chart is really two separate charts put into one so it is confusing. The red lines and the black lines are totally different so intersection means nothing. The red lines show the power consumption (in killowatts) compared to flow rate (in GPM). The black lines show the pressure head (in feet of water) vs. the flow rate.
Basically, the slower speeds (RPM) are more energy efficient at a faster rate than the way that RPM and GPM vary. Remember that I said that at least approximately, RPM and GPM vary directly together so that if you cut the RPM in half you roughly cut the GPM in half. Well, for power consumption, the very rough rule is that cutting the RPM in half cuts the power consumption by one-eighth. So lower RPM is almost always more efficient in terms of converting electricity into a flow rate (the efficiency in terms of output power is about the same but output power is the product of flow rate with pressure).
The point I was making was that at some very low RPM this savings goes away because there is a fixed electrical cost even at 0 GPM and that appears to be around 80 Watts. So at very low RPM you don't get the same sort of savings as at higher RPM and in fact at some point you stop getting savings. Let's just make some very simple assumption to see how this might work using the following very rough formulas:
GPM = 0.03 * RPM
Electrical Input Watts = 80 + (RPM/250)^3
You want to achieve one turnover which is a fixed number of gallons (G) so the total power consumption for one turnover is:
Energy Per Turnover (KWh-per-1000 gallons) = Kilowatts / GPM / 60 = ( 80 + (RPM/250)^3 ) / (1.8 * RPM)
You want to find a minimum for this function which clearly goes down with lower RPM for a while, but at some point starts to go back up again. Though we could take the derivative to find this out, it's instructive to just look at a table of values to see how this roughly changes:
So you can see that the minimal amount of energy is consumed around 750-1000 RPM though 600-1200 RPM is still very efficient. However, if you have an extremely large pool, then you will be limited by the 24 hour turnover requirement. However, the sweet spot for best energy efficiency will handle pools up to around 50,000 gallons in size which is far larger than most residential pools. So running at 750-1000 RPM is best and would turnover a 15,000 pool in around 8-11 hours.
RPM KWh-per-1000gal GPM Turnover-Time-per-1000gal (Hours) Max. Size (gallons) for 24 hours
3450 0.44 104 0.16 150,000
2000 0.16 60 0.28 86,000
1500 0.11 45 0.37 65,000
1200 0.09 36 0.46 52,000
1000 0.08 30 0.56 43,000
750 0.08 23 0.74 32,000
600 0.09 18 0.93 26,000
500 0.10 15 1.11 22,000
250 0.18 8 2.22 11,000
This is an interesting analysis that I was going to have to do anyway for my own pool and it shows that using extremely slow pump speeds to achieve 24 hour turnover is NOT the smartest thing to do for optimum energy usage.
The only way to really confirm all of this is to take your actual pump in your real pool and measure its energy usage at different RPM and compare against the total energy consumed by looking at the GPM flow rate and the size of your pool for one turnover. However, I'll bet that my equation assumptions aren't that far off. It would be interesting to find out, though. Be sure and keep us posted (if you do get a flow meter since that's the only way you'll know how long you truly need to run the pump for one turnover).