ERROR: ReadOnlyMemoryError()

After a recent Pkg.update() I get the follwing error

```
_
_ _ _(_)_ | A fresh approach to technical computing
(_) | (_) (_) | Documentation: https://docs.julialang.org
_ _ _| |_ __ _ | Type "?help" for help.
| | | | | | |/ _` | |
| | |_| | | | (_| | | Version 0.6.0 (2017-06-19 13:05 UTC)
_/ |\__'_|_|_|\__'_| | Official http://julialang.org/ release
|__/ | x86_64-pc-linux-gnu

julia> using Plots

julia> pyplot()
Plots.PyPlotBackend()

julia> plot(rand(100))
Error showing value of type Plots.Plot{Plots.PyPlotBackend}:
ERROR: ReadOnlyMemoryError()
julia>
```
Typing control-d after that results in a segmentation fault.

Reproduce the report with Julia 0.6 by running `using Plots`, `pyplot()`, and `plot(rand(100))`. Investigate the resulting `ReadOnlyMemoryError` and the segmentation fault after Ctrl-D; done means the plotting command and exit path no longer produce these failures.

由索引模型根据 Issue 内容生成。