The LM3914 might not be the ideal way to drive LEDs from an arduino. It is a complete A-to-LED driver unit designed to take an analogue input (say 0 to X volts) and drive a range of 10 LEDS with LED1 lighting at 0.1X, LED2 at 0.2X etc with LED10 lighting at X
The output of the arduino is a PWM 5 volt signal. If you want to create a 0-5 analogue voltage from that to control the 3914 you'll have to create an R-C smoothing system and then use that smoothed output to drive the LM3914. Generally the 3914 switching of LEDs is not hard-off to hard-on, there is a sort of deadband where the LEDs go through a transition range from off to on. As the input moves from one transition point to the next, the next LED starts to glow dimly and isn't full on until the cross-over deadband has been exceeded.
You might be better to use a couple of binary LED drivers to drive 16 LEDs. That way you switch the LEDs under software control and each will be either full off or full on. If you really need 20 then use 3 driver units.
jack