If you want to try to make your own graphs (say in Excel) then Ed Williams' Aviation Formulae is a great source (In fact a goldmine). I've used it often.
Aviation Formulary V1.43
By Ed Williams
With full respect for his rights, I here's a taster of what you can find there.
Specifically, he has this formula to calculate RH. Of course multiply by 100 for a percentage.
f= exp(17.27(Td/(Td+237.3)-T/(T+237.3)))
or inverted to find dewpoint.
Td=237.3/(1/(ln(f)/17.27+T/(T+237.3))-1)
A related formula gives the increase in effective density altitude due to humidity. It only addresses the reduction of air density, and not the effect on engine power output:
Increase(ft)=0.267*RH*(T+273)*exp(17.3*T/(T+237))*(1-0.00000688*H)^(-5.26)
RH (f above) is the relative humidity expressed as a fraction, T is the temperature in Celsius and H is the pressure altitude in feet.
Examples are:
SL/30C/100% -> 565' increase in DA
10000/5C/80% -> 124' increase in DA
5000/40C/80% -> 977' increase in DA.
In terms of the dewpoint, Td the formula is:
Increase(ft)=0.267*(T+273)*exp(17.3*Td/(Td+237))*(1-0.00000688*H)^(-5.26)
which clearly agrees with the above when T=Td and RH=1.
Have fun.
cl12pv2s