No, it is not.
A function is linear if graph of that function is straight line. If you try to graph tan(x) you will see it is far from straight line.
take several values of x and calculate tan(x) and than draw a graph.
check how tan(x) looks on picture.
the reason why there is multiple curve lines is because angle (x axis) can be larger than full circle (2pi radians) or go negative ( -2pi radians and less)
note that x axis is presented in radians not degrees!
