Question

I am new to using matplotlib. I am trying to create a 2D grid, using matplotlib. This is my first time using matplotlib for anything non trivial.

I have decided to break the task into 3 parts:

1. Create the grid table (shown below), color the appropriate columns, and label the axis correctly. This is the one I need most help with. My initial idea is to hold the data for the table in a list of dictionaries (or list of lists); the data struct could hold some meta data about which columns were colored, and then I could simply create the matplot plot off of that data – but I haven’t really done any plotting with matplotlib and could do with some help in getting started.

2. Plot a symbol (say ‘X’) in a grid cell with coordinates (row,col)

3. Save the grid table as a picture (this one is easy, I can do by myself)

Here is a picture of the kind of grid table I am looking to create using matplotlib:

I will be very grateful for any help that gets me started.

PS: the image is not rendering very well. The horizontal lines in the table are all the same weight (i.e. thickness), so the visual effect of the grid table should look like an Excel worksheet.

[[Edit/Update]]

Just to clarify, what I’m trying to create is a kind of ‘chess like’ game board. I have managed to modify the code snippet Ricardo posted in his answer, to get as close as I can (with my limited matplotlib skills!) to this game board as I can. However, there are a couple of things “missing”:

1. The x axis column labels are strings, not numbers, they are string labels e.g. AB1, AB2, AB3 etc. Additionally, these labels are midpoints (i.e. they are centered, or lie BETWEEN the x axis ticks – not on the ticks themselves)

2. I need to be write symbols in a particular column for a given y axis value, for example, I may want to write the text ‘foo’ at y axis value -1565.5 in column ‘AB2’.

Once I have this, I am sure I will be able to hack something together to get to the game I am trying to write – especially, since I have just bought a copy of Matplotlib for Python developers.

Answer 1

Mmh… I guess you could achieve this by asking matplotlib to show the grid, and combining a barplot (to color the columns) with a scatter plot or direct text drawing, for the symbol(s)

Edit: This may help you getting started. Needs some work on the ticks, though.

#!/usr/bin/python

from pylab import *
import matplotlib
import matplotlib.ticker as ticker

# Setting minor ticker size to 0, globally.
# Useful for our example, but may not be what
# you want, always
matplotlib.rcParams['xtick.minor.size'] = 0

# Create a figure with just one subplot.
# 111 means "1 row, 1 column, 1st subplot"
fig = figure()
ax = fig.add_subplot(111)
# Set both X and Y limits so that matplotlib
# don't determine it's own limits using the data
ax.set_xlim(0, 800)

# Fixes the major ticks to the places we want (one every hundred units)
# and removes the labels for the majors: we're not using them!
ax.xaxis.set_major_locator(ticker.FixedLocator(range(0, 801, 100)))
ax.xaxis.set_major_formatter(ticker.NullFormatter())
# Add minor tickers AND labels for them
ax.xaxis.set_minor_locator(ticker.AutoMinorLocator(n=2))
ax.xaxis.set_minor_formatter(ticker.FixedFormatter(['AB%d' % x for x in range(1, 9)]))

ax.set_ylim(-2000,6500, auto = False)
# And set the grid!
ax.grid(True, linestyle='-')

# common attributes for the bar plots
bcommon = dict(
    height = [8500],  # Height = 6500 - (-2000)
    bottom = -2000,   # Where to put the bottom of the plot (in Y)
    width = 100)      # This is the width of each bar, itself
                      # determined by the distance between X ticks

# Now, we create one separate bar plot pear colored column
# Each bar is a rectangle specified by its bottom left corner
# (left and bottom parameters), a width and a height. Also, in
# your case, the color. Three of those parameters are fixed: height,
# bottom and width; and we set them in the "bcommon" dictionary.
# So, we call bar with those two parameters, plus an expansion of
# the dictionary.

# Note that both "left" and "height" are lists, not single values.
# That's because each barplot could (potentially) have a number of
# bars, each one with a left starting point, along with its height.
# In this case, there's only one pair left-height per barplot.
bars = [[600, 'blue'],
        [700, 'orange']]
for left, clr in bars:
    bar([left], color=clr, **bcommon)

show()