plotting phase of a signal adding delay
This is indeed correct. There is a few things you could do to make it a better graph:


Label the X-axis
Label the Y-axis
Use real physical units if it's applicable. Phase is measured either in radians or in degrees. These are VERY different things, so proper labeling with units helps to clarify what you are using. Similar if f is a frequency in Hz (or kHz or MHz) and T is a time (in nanoseconds or fortnights), it should be stated.
You could consider changing the Y-grid so that the grid lines match with meaningful phase values such as 2*pi (or an integer multiple thereof).
Along the same lines, you could alternatively normalize the phase to a meaningful value (like 2*pi) and keep the integer grid.
If the result was obtained by Fourier transform of a real valued time domain signal and the context is clear, consider omitting the left half (f&lt;0) of the plot. The convention is to only plot positive frequencies and the complex conjugate symmetry is implied. 


This may sound nit-picky, but making a good graph is typically time well spend. It really helps to clarify what's going on, simplifies communication and will help you to remember what you did when you look at it in 3 months from now.