Bar plots can't accept nan values.

**Problem Description**

Application crash caused by uncaught C++ exception originating from `enable/kiva/agg` (presumably; origin not confirmed by debug or error trace). It might be more appropriate to file this issue against `enable`, but I thought I would start here.

The error message:
```
libc++abi.dylib: terminating with uncaught exception of type std::overflow_error: Exceeded cell block limit
Abort trap: 6
```

**Reproduction Steps:**

Run the following test code.

```python
import numpy as np

from chaco.api import ArrayPlotData, Plot
from enable.api import ComponentEditor
from traits.api import HasTraits, Instance
from traitsui.api import UItem, View

class Test(HasTraits):
bar_plot = Instance(Plot)

def _bar_plot_default(self):
x = [0, 1]
y = [np.nan, 1.0]
plot = Plot(ArrayPlotData(x=x, y=y))
plot.plot(('x', 'y'), type='bar')
return plot

traits_view = View(
UItem('bar_plot', editor=ComponentEditor()),
)

if __name__ == "__main__":
Test().configure_traits()
```

**Expected behavior:**

Open a QT window with Chaco bar plot.

**OS, Python version:**
macOS 10.14.6

```
$ python
Enthought Deployment Manager -- https://www.enthought.com
Python 3.6.0 |Enthought, Inc. (x86_64)| (default, Mar 3 2017, 03:26:01)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin
```

Package versions:
traits 5.1.2-1
traitsui 6.1.1-3
chaco 4.7.2-4
enable 4.8.0-1
pyqt 4.12.1-1