Keeping line segments straight: flat parameter

The flat parameter controls how line segments are handled by a non-Cartesian coordinate system or map projection.

• flat=False (the default) allows the coordinate system or projection to transform the line, so it may appear curved.

• flat=True keeps line segments straight in plot/projected coordinates.

On Mercator/livemap this means a straight projected line, not a geodesic shortest path (see also an example). flat is not livemap-only; it is also useful with non-Cartesian coordinate systems such as coord_polar().

from lets_plot import *
from lets_plot.geo_data import *

The geodata is provided by © OpenStreetMap contributors and is made available here under the Open Database License (ODbL).

LetsPlot.setup_html()

Map coordinates

route = {
    'lon': [-156.78861, -99.12766],
    'lat': [71.29056, 19.42847],
}

countries = geocode_countries(names=["US", "Canada", "Mexico"]).inc_res().get_boundaries()

gggrid([
    (ggplot()
      + geom_map(data=countries, size=.25)
      + geom_path(aes(x="lon", y="lat"),
                  data=route, color="white", size=1,
                  manual_key=layer_key("default"))
      + geom_path(aes(x="lon", y="lat"),
                  data=route, color="coral", size=1, flat=True,
                  manual_key=layer_key("flat=True"))
      + coord_map(xlim=[-180, -50], ylim=[15, 75])
      + theme_void() + theme(legend_position='bottom') + flavor_darcula()
      + ggtitle("coord_map()")),
    (ggplot()
      + geom_livemap(tiles=maptiles_lets_plot(theme='dark'))
      + geom_path(aes(x="lon", y="lat"),
                  data=route, color="white", size=1,
                  manual_key=layer_key("default"))
      + geom_path(aes(x="lon", y="lat"),
                  data=route, color="coral", size=1, flat=True,
                  manual_key=layer_key("flat=True"))
      + theme(legend_position='bottom') + flavor_darcula()
      + ggtitle("geom_livemap()"))
])

Polar coordinates

data = {
    "x": [0, 1, 1, 0, 0],
    "y": [1, 1, 3, 3, 1],
}

(ggplot(data, aes("x", "y"))
  + geom_path(color="white", size=2,
              manual_key=layer_key("default"))
  + geom_path(color="coral", size=1, flat=True,
              manual_key=layer_key("flat=True"))
  + coord_polar(xlim=[0, 4], ylim=[0, 4])
  + theme_minimal2() + theme(legend_position='bottom', axis='blank') + flavor_darcula()
  + ggtitle("coord_polar()")
  + ggsize(500, 500))