Make the value of R1 very small relative to the value used for R3 to R12. You probably won't like the magnitude of the maximum voltage across R1, but look at the shape of curve. Think about why it is different.
Something else you could try:
Make the value of R2 quite small and operate in bar mode. R2 needs to be low enough in value that when all of the LEDs are on they are all able to operate properly in constant current mode. Disconnect all the other resistors. Plot the voltage across R2 as the input voltage to the 3914 is changed.
Think about what Kirchhoff's voltage law and current law tell you.
Something else you could try:
Make the value of R2 quite small and operate in bar mode. R2 needs to be low enough in value that when all of the LEDs are on they are all able to operate properly in constant current mode. Disconnect all the other resistors. Plot the voltage across R2 as the input voltage to the 3914 is changed.
Think about what Kirchhoff's voltage law and current law tell you.