NCL_bar_4.py

This script illustrates the following concepts:

• Drawing bars instead of curves in an XY plot

• Changing the aspect ratio of a bar plot

• Turning off the outline of filled bars in a bar plot

• Drawing filled bars up or down based on a Y reference value

• Setting the minimum/maximum value of the Y axis in a bar plot

See following URLs to see the reproduced NCL plot & script:

• Original NCL script: https://www.ncl.ucar.edu/Applications/Scripts/bar_4.ncl

• Original NCL plot: https://www.ncl.ucar.edu/Applications/Images/bar_4_lg.png

Import packages

import numpy as np
import xarray as xr
import matplotlib.pyplot as plt

import geocat.datafiles as gdf
import geocat.viz as gv

Read in data:

# Open a netCDF data file using xarray default engine and load the data into xarrays
ds = xr.open_dataset(gdf.get("netcdf_files/soi.nc"))
dsoik = ds.DSOI_KET
date = ds.date
num_months = np.shape(date)[0]

# Dates in the file are represented by year and month (YYYYMM)
# representing them fractionally will make ploting the data easier
# This produces the same results as NCL's yyyymm_to_yyyyfrac() function
date_frac = np.empty_like(date)
for n in np.arange(0, num_months, 1):
    yyyy = int(date[n] / 100)
    mon = (date[n] / 100 - yyyy) * 100
    date_frac[n] = yyyy + (mon - 1) / 12

Plot:

# Generate figure (set its size (width, height) in inches) and axes
plt.figure(figsize=(12, 6))
ax = plt.axes()

# Create a list of colors based on the color bar values
colors = ['red' if (value > 0) else 'blue' for value in dsoik[::8]]

# Plot bar chart
plt.bar(date_frac[::8],
        dsoik[::8],
        align='edge',
        edgecolor="none",
        color=colors,
        width=8 / 12)

# Use geocat.viz.util convenience function to add minor and major tick lines
gv.add_major_minor_ticks(ax,
                         x_minor_per_major=4,
                         y_minor_per_major=5,
                         labelsize=20)

# Use geocat.viz.util convenience function to set axes parameters
gv.set_axes_limits_and_ticks(ax,
                             ylim=(-3, 3),
                             yticks=np.linspace(-3, 3, 7),
                             yticklabels=np.linspace(-3, 3, 7),
                             xlim=(date_frac[40], date_frac[-16]),
                             xticks=np.linspace(1900, 1980, 5))

# Use geocat.viz.util convenience function to set titles and labels
gv.set_titles_and_labels(ax,
                         maintitle="Darwin Southern Oscillation Index",
                         ylabel='Anomalies',
                         maintitlefontsize=28,
                         labelfontsize=20)

plt.tight_layout()
plt.show()