How to read a neuron tuning curve graph?
From the comments:

I'm going to hazard a guess that these are neurons that are tuned to a particular direction in space and that the x-axis is the angle in multiples of π radians, particularly since these are related to the work of Georgopoulos and colleagues.

Since we know these are positionally tuned neurons, you can see some other examples in 
this Wikipedia page for some other examples.


  A typical population code involves neurons with a Gaussian tuning curve whose means vary linearly with the stimulus intensity, meaning that the neuron responds most strongly (in terms of spikes per second) to a stimulus near the mean.


So, as mentioned above, the independent variable here is the stimulus "intensity" (in this case abstracted by the quantity of direction) and the firing rate is the dependent variable, hence their positions on the axes in the plot.

For some of the more theoretical studies of this type of phenomenon, I would definitely check out the work of Emo Todorov, among many others, as well.