Problems with Basemap.scatter

还没有人认领这个 Issue。

Hi,

I have stumbled over a bug in scatter plot. With latlon=True, it crashes with zero, one or two points.

Example code:

``` python
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

bmap = Basemap(projection='merc',
llcrnrlon=9, urcrnrlon=13,
llcrnrlat=63, urcrnrlat=65,
resolution='c')

lon = [10.0, 12.0]
lat = [64.0, 65.0]

bmap.scatter(lon, lat, latlon=True)

plt.show()
```

This crashes with the following Traceback:

```
Traceback (most recent call last):
File "a.py", line 12, in
bmap.scatter(lon, lat, latlon=True)
File "/opt/anaconda/lib/python2.7/site-packages/mpl_toolkits/basemap/__init__.py", line 536, in with_transform
x = self.shiftdata(x)
File "/opt/anaconda/lib/python2.7/site-packages/mpl_toolkits/basemap/__init__.py", line 4775, in shiftdata
thresh = 360.-londiff_sort[-2]
IndexError: index -2 is out of bounds for axis 0 with size 1
```

I am using python 2.7.10, matplotlib 1.4.3, basemap 1.0.7 on a 64-bit Linux machine

首先运行提供的 Basemap.scatter 示例，并使用 latlon=True，然后检查 mpl_toolkits/basemap/__init__.py 中的 shiftdata 入口点，该处的 traceback 在输入较小时失败。完成标准是 scatter 能够处理零个、一个和两个点而不引发 IndexError。

由索引模型根据 Issue 内容生成。