I just added some more description for the simulation.
Normally, the drag force is very small for small projectile motion in air.
So the initial velocity was set to 626 m/s (very large value) and the number display for distance was always in unit of km, so that the air drag effect can be visible more easily.
The right most text field (initial value=100) is used for scaling the X-Y coordinate.
For example: You want to have a smaller initial velocity v=62.6 m/s.
You might want to change the sclae from 100. to 1. (The same trajectory will be displayed for v=626m/s and scale=100.)
If you want to have smaller velocity, i.e. v=20 m/s , you might want to change scale to 0.1
The k value for the simulation should always be positive. (The negative sign already entered in the code)
Because you enter a negative k value, it was simulating some push force was added(instead of drag force).
Please check the unit of your formula. Why the density is 1.25 ? This is not the density of air (also check the unit for the density in your formula)!