"""
=================================================
Drawing heliographic longitude and latitude lines
=================================================

This example demonstrates how you can draw individual Stonyhurst
longitude and latitude lines.
"""
import matplotlib.pyplot as plt
import numpy as np

import astropy.units as u
from astropy.coordinates import SkyCoord

import sunpy.map
from sunpy.coordinates import frames
from sunpy.data.sample import AIA_171_IMAGE

###############################################################################
# We will first demonstrate the coordinate transformations that occur under the
# hood to show a heliographic grid line of longitude or latitude.
# Let's start with a map using the sample data.

aia = sunpy.map.Map(AIA_171_IMAGE)

###############################################################################
# We can define a heliographic coordinate for a single point.

lon_value = 35 * u.deg
lat_value = 12 * u.deg
stonyhurst_frame = frames.HeliographicStonyhurst(obstime=aia.date)
point_in_stonyhurst = SkyCoord(lon=lon_value, lat=lat_value, frame=stonyhurst_frame)

###############################################################################
# Next we transform it into the coordinate frame of our map, which is helioprojective.

point_in_hpc = point_in_stonyhurst.transform_to(aia.coordinate_frame)
print(point_in_hpc)

###############################################################################
# Now let's define two lines, one of constant longitude and one of constant latitude,
# that pass through the previously defined coordinate point.
# We don't need to explicitly transform them to the coordinate frame of our
# map because they will be transformed automatically when plotted.

num_points = 100
constant_lon = SkyCoord(lon_value, np.linspace(-90, 90, num_points) * u.deg,
                        frame=stonyhurst_frame)
constant_lat = SkyCoord(np.linspace(-90, 90, num_points) * u.deg, lat_value,
                        frame=stonyhurst_frame)

###############################################################################
# Now let's plot the single coordinate point and the individual lines of
# constant longitude and latitude. We'll overlay the autogenerated lon/lat
# grid as well for comparison.

fig = plt.figure()
ax = fig.add_subplot(projection=aia)
aia.plot(axes=ax, clip_interval=(1, 99.99)*u.percent)
ax.plot_coord(constant_lon, color="lightblue")
ax.plot_coord(constant_lat, color="tomato")
ax.plot_coord(point_in_stonyhurst, marker="o")
aia.draw_grid(axes=ax)

plt.show()