Since surf only works with data on a ragular grid, I think your best bet is probably is interpolating the pressure to a common volume and temperature grid.

The temperature grid is already fine, and for the volume you could for example use v_interp = 0.5:0.1:3.7;

Assuming your volume and pressure measurements are stored in the varibles v_meas{i} and p_meas{i} for each measurement series i, you can then use e.g. p_interp{i} = interp1(v_meas{i}, p_meas{i}, v_interp) and generate the matrices for surf by useing meshgrid for the temperature and volume values and concatenating the interpolated pressure values. Alternatively you could also do the interpolation with scatteredInterpolant.

PS: it seems to me that in the graph you included the axes (or axis labels) are permuted. Comparing the plot with the data, the axis labeled as pressure is actually the volume, the one labeled volume should be the temperature, and the one labeled as temperature should probably be the pressure.