Bear with me - it's been a while since I did this at school!
I need to plot a curve in the form of a square root (kind of an 'r' shape if you will)
I have 6 intervals along my x axis, and my maximum y value can be 180. Given a value of x, how can I work out my value of y?
Many thanks
If I understand well your question, you want a function that returns a 'r' like shape between $0$ and $x_{max}$ and in a range from $0$ to $y_{max}$.
To do so you can just use: $$y(x) = \frac{\sqrt{x}}{\sqrt{x_{max}}}*y_{max}$$
Here your $y_{max}=180$ so you can use: $$y(x) = \frac{\sqrt{x}}{\sqrt{x_{max}}}*180$$
This should give you the shape you expect.
Here (see comments) are the plots for different functions shaped like you asked.