runtime warning for add-edge

For the following code:

```
pgm = daft.PGM(dpi = 150, alternate_style="outer")

pgm.add_node("sb","Start\nBalance", 1, 4, aspect = 2.2, observed = True, plot_params = {'facecolor': 'cadetblue'})
pgm.add_node("ba","Bet\nAmount", 1, 3, aspect = 2.2, shape = "rectangle", plot_params = {'facecolor': 'thistle'})

pgm.add_edge("sb", "ba")
pgm.render()
```

I get a runtime warning error message:
```
/usr/local/lib/python3.7/dist-packages/daft.py:843: RuntimeWarning: divide by zero encountered in double_scalars
* (np.sign(dy) or 1.0)

```


The graph renders just fine, but I am unsure whether I am doing something wrong or if the Warning should not really occur. Thanks!