Couldn't find this basic equation on here so I thought I would add here.
To caculate how much lp is needed to get desired level
x is desired skill level
z is current skill level
y is needed lp
y=(50x^2+50x)-(50z^2+50z)
eg
y=(50(5)^2+50(5))-(50(1)^2+50(1))
y=1400
If you want to graph, only fill in current skill level.
Enjoy
edit: y=(50x^2+50x-100)-(50z^2+50z-100) to y=(50x^2+50x)-(50z^2+50z)