error plotting points that wrap around longitudinal map boundaries in plot() method of Basemap

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
matplotlib, python
Domain
data-visualization

Research direction

Reproduce the failure with the supplied Python script, then inspect mpl_toolkits/basemap/init.py at the plot wrapper and shiftdata method around the reported line. The fix is done when the unsorted [-30, 10, -20] points plot without the ambiguous-array error while preserving their order.

Written by the indexing model from the issue text.

Description

Hi

Using Python 2.7.8 and Basemap 1.0.7 there seems to be a problem plotting points that wrap around longitudinal map boundaries in plot() method of Basemap if the points wrap back and forth. The problem can be avoided by sorting the points by longitude prior plotting (but this does of course not work if plotting a segment based on the points since sorting will screw up the order of the points along the segment)

The following code illustrates the problem by plotting the same three points sorted in different ways (first plot works)

#! /usr/bin/env python

import matplotlib
matplotlib.use('TkAgg')
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

bm = Basemap(llcrnrlon=0,llcrnrlat=-80,urcrnrlon=360,urcrnrlat=80,projection='mill')
fig=plt.figure(figsize=(8,4.5))   

# figure with three points work
bm.plot([-30,-20,10], [10,10,10], "ro", latlon=True)
plt.show()

# figure with less than three points fail
bm.plot([-30,10,-20], [10,10,10], "ro", latlon=True)
plt.show()

and the error running the above code (after closing the first successful plot window) is

:~/SNSN/ALERT/seedlink> python TryBasemap.py 
Traceback (most recent call last):
  File "TryBasemap.py", line 16, in <module>
    bm.plot([-30,10,-20], [10,10,10], latlon=True)
  File "[...]/python2.7/site-packages/mpl_toolkits/basemap/__init__.py", line 536, in with_transform
    x = self.shiftdata(x)
  File "[...]/python2.7/site-packages/mpl_toolkits/basemap/__init__.py", line 4777, in shiftdata
    if itemindex:
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

regP

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