The frontend would like to store viewport information for each cell so
that visualizations are zoomed to the proper extents upon rendering.
This adds a property to cells called "axes" which takes the following
shape:
```
{
"axes" : {
"y" : {
"bounds" : [
0,
2
]
},
"y2" : {
"bounds" : [
1,
3
]
}
}
}
```
Bounds specify the visible range for the axis, and are a 2-tuple of the
form [lower, upper]. Bounds are not implicitly inclusive or
exclusive--that determination is left for clients to make. Also, there
are no restrictions on the naming of axes.
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