transfer_functions.shape raise ValueError or locate at wrong coordinate when zooming (using bokeh as plotting frontend)

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## Description

Things to consider including:

* What went wrong, or what were you unable to do?
I want to zoom the plotting and use ds handle the data to reduce js workload. However sometimes ValueError is raised, or the points always located at bottom left hand corner

* What did you expect to happen?
Zoom -> ds handle data -> plot the graph normally at right coordinate

* A screen shot, short screen cast, or sketch.

## Errors and warnings

If relevant, please include:

* Python tracebacks and/or warnings.
https://gist.github.com/phillui-37/cc6509878d0fd5afe7d23dd9eeacaee8#file-error-stack-trace

* Any messages/tracebacks in your browser's developer tools console.

## How to reproduce

Ideally, please include a complete, minimal, self-contained python script or notebook that reproduces the issue (in the environment you describe below). http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports contains useful guidance on doing this if you are unsure.
[Src code here](https://gist.github.com/phillui-37/cc6509878d0fd5afe7d23dd9eeacaee8)

Just run the flask, open test.html and zoom the plotting

## Your environment

If your version of datashader is neither the latest release (https://github.com/bokeh/datashader/releases/latest) nor the latest GitHub master, please consider upgrading before filing your issue.

Please tell us how to reproduce your environment:

* If using conda, the output of `conda list` and `conda info`.
https://gist.github.com/phillui-37/cc6509878d0fd5afe7d23dd9eeacaee8#file-req-txt

* If using pip, the output of `pip freeze`.

* What operating system and version?
OSX Mojave 10.14.2

* If relevant, which browser and version?
Firefox 64.0
Chrome 71.0.3578.98