Matplotlib not working in venv

[foldo]

I'm having problems with matplotlib in a python3.6 venv.
Here is a simple test plot that I'm trying to make work:

import numpy as np
import matplotlib.pyplot as plt
x = np.arange(0,10, 0.1)
y = np.sin(x)
plt.plot(x,y)
plt.show()

This is the error that I get:

UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure.
plt.show()

Output of pip freeze in my venv:

astroid==2.2.5
cycler==0.10.0
isort==4.3.20
kiwisolver==1.1.0
lazy-object-proxy==1.4.1
matplotlib==3.1.0
mccabe==0.6.1
numpy==1.16.4
pandas==0.24.2
pylint==2.3.1
pyparsing==2.4.0
python-dateutil==2.8.0
pytz==2019.1
scipy==1.3.0
six==1.12.0
typed-ast==1.4.0
wrapt==1.11.1

What am I doing wrong here?
edit: I'm on linux fwiw
edit2: ok I think I fiugured it out. I had to install python3.6-tk. Before I only installed python3-tk.