I'd recommend disregarding the graphs for the moment and just learning the relationship: -
(1) Pressure drops as you climb
(2) Below the tropopause at 36,069 ft, temperature drops as you climb. Except for an inversion, where the relationship inverts, or an isothermal layer, where it does nothing.
Density = sea level density * relative pressure / relative density.
Local speed of sound is proportional to the square root of local absolute temperature. So, if you take speed of sound at sea level, and multiply it by SQRT [ (S/L OAT in °C + 273.15) / (local OAT in °C + 273.15) ] you'll have your local speed of sound.
Now to the actual speeds...
(1) IAS = what you see on the dial
(2) CAS or RAS = IAS corrected for system pressure errors. There's a graph in the POH to show you the errors, and in any case it should not be more than 5 knots different on a certified aeroplane.
(3) EAS = CAS corrected for compressibility. Below FL100 and M=0.7 you can assume EAS=CAS with no correction needed.
(4) TAS = EAS corrected for altitude. The correction is TAS = EAS divided by the square root of relative density.
(5) Mach number = TAS divided by local speed of sound.
Personally I don't use graphs in working such stuff out (and as you'll guess from the fact that I've just written all this from memory) I use ISA tables, or if I'm being very accurate, the basic ISA equations.
G