NCL_xy_13.py

This script illustrates the following concepts:

• Adding error bars on an XY plot

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

• Original NCL script: https://www.ncl.ucar.edu/Applications/Scripts/xy_13.ncl

• Original NCL plots: https://www.ncl.ucar.edu/Applications/Images/xy_13_lg.png

Import packages:

import numpy as np
import xarray as xr
import matplotlib.pyplot as plt
from matplotlib import collections as mc

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/uv300.nc"))

# Extract data
V = ds.isel(time=0, lon=30, drop=True).V

# Create arrays to represent the magnitude of error above and below the line
# This data is arbitrary and you should replace these arrays with the actual
# error for your dataset
err_above = V.data + 1.5
err_below = V.data - 1
x = range(0, 64)  # the x values used to plot the data and error bars

# Make a tuple to represent the bottom and top points of the error bar
err_below = tuple(zip(x, err_below))
err_above = tuple(zip(x, err_above))

# Make a tuple containing those points to describe the line segment
segments = tuple(zip(err_below, err_above))

# Create a line collection so we can plot all of the segments with one call
bars = mc.LineCollection(segments, colors='black', linewidths=0.5)

Plot:

plt.figure(figsize=(8, 8))
ax = plt.axes()

# Plots the data with markers
plt.plot(x, V.data, color='black', linewidth=0.5, marker='.')

# Plot the error bars
ax.add_collection(bars)

# Use geocat.viz.util convenience function to set axes parameters
gv.set_axes_limits_and_ticks(ax,
                             xlim=(0, 70),
                             ylim=(-9, 9),
                             xticks=np.arange(0, 71, 10),
                             yticks=np.arange(-9, 10, 3),
                             yticklabels=np.arange(-9.0, 10.0, 3.0))

# Use geocat.viz.util convenience function to add minor and major tick lines
gv.add_major_minor_ticks(ax,
                         x_minor_per_major=5,
                         y_minor_per_major=3,
                         labelsize=14)

# Use geocat.viz.util convenience function to set titles and labels
gv.set_titles_and_labels(ax,
                         maintitle="Example of error bars",
                         ylabel=V.long_name + " " + V.units)

plt.show()