That IS the graph of a tangent function....specifically tan (x/3)
The period is the distance on the x-axis before the graph repeats itself
3pi/2 - - 3pi/2 = 3pi
Here is a desmos graph of tan (x/3)
https://www.desmos.com/calculator/ny77s8o78x
For comparison.....here is a graph of tan x
https://www.desmos.com/calculator/t4yhf9vfzq