Here's what i'm trying to do:

```
# the standard broiler plate for jupyter
%matplotlib inline
from matplotlib.pyplot import *
from sympy import *
import numpy as np
init_printing()
t = symbols('t')
x1 = 2*cos(3*pi*t + pi/4)
x2 = x1.diff(t)
display(x1)
display(x2)
pi2 = 2*np.pi
vt = np.arange(0, pi2, 0.1)
vx1 = np.zeros(len(tv))
vx2 = np.zeros(len(tv))
for n in range(0,len(vt)):
vx1[n] = N(x1.subs(t,vt[n]))
vx2[n] = N(x2.subs(t,vt[n]))
title('Signal')
plot(vt, vx1)
show()
title('Derivative of signal')
plot(vt, vx2)
show()
```

The above seems like it should work... however, I'm getting unexpected error message:

```
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-41-eaccf9c2bc6b> in <module>()
14
15 title('Signal')
---> 16 plot(vt, vx1)
17 show()
18 title('Instantanious Frequency')
C:\Python35\lib\site-packages\sympy\plotting\plot.py in plot(*args, **kwargs)
1289 series = []
1290 plot_expr = check_arguments(args, 1, 1)
-> 1291 series = [LineOver1DRangeSeries(*arg, **kwargs) for arg in plot_expr]
1292
1293 plots = Plot(*series, **kwargs)
TypeError: 'NoneType' object is not iterable
```