I am trying to plot two 1-D masked arrays against each other in a line plot and an extraneous straight line appears on the plot. This phenomenon only occurs sporadically and with certain data sets. I have noticed a similar phenomenon with masked arrow arrays, but that is much harder to track down. The masked elements are intended to break the plot line so that several independent polylines are plotted. (The purpose is to plot a map of coastal outlines.)
I am attaching a python script which reproduces the problem,
but only with a particular data set, which is also attached.
Sorry, if I try to shorten the data set more than I have
already, the problem goes away, even if I split the file
in half an plot each half separately!
I am running on a 32 bit intel processor using debian testing
and the numpy and matplotlib versions are 1.3 and 0.99.1.2.
However, the problem also appears on a 64 bit amd processor
running debian stable with numpy and matplotlib versions
1.3 and 0.99.1.1.
The python script is named maskbug.py and the data set is
trunc1.dat, which is an ascii file. The data set should be
read on the standard input, i. e.,
maskbug.py < trunc1.dat
I have verified by printing the masked arrays that nothing
appears to go wrong in the conversion from ascii to numpy
masked array.
Dave Raymond
Physics Dept.
New Mexico Tech
Socorro, NM 87801
#!/usr/bin/python
# this illustrates a bug which occurs when plotting masked data
import sys
from numpy import *
import matplotlib.pyplot as plt
# get lon-lat data from standard input and store in lists
lonl = []
latl = []
while 1:
inline = sys.stdin.readline()
if inline == '':
break
else:
pieces = inline.split()
lonl.append(float(pieces[0]))
latl.append(float(pieces[1]))
# convert lists to numpy arrays
lon = array(lonl)
lat = array(latl)
# compute masks -- masked elements have values of 1.e30
masklon = lon > 0.999e30
masklat = lat > 0.999e30
# compute masked arrays
mlon = ma.array(lon, mask = masklon)
mlat = ma.array(lat, mask = masklat)
# list input file
print mlon
print mlat
# plot them
cc = plt.plot(mlon,mlat)
ax = plt.axis([120,150,0,30])
plt.show()
trunc1.dat
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