graph not showing when I set format to line

pip -> matplotlib version 1.5.3
anaconda -> python 3.5.2

Only shows an empty graph

``` python
plt.plot_date(x=[X['Date'][400:500]], y=[X['Last Traded'][400:500]], fmt='r-')
plt.show()
```

Shows graph but with 'o' instead of a line

``` python
plt.plot_date(x=[X['Date'][400:500]], y=[X['Last Traded'][400:500]])
plt.show()
```
