Re: [Matplotlib-users] [ANN] basemap matplotlib toolkit 0.1.1

[Ted D. <ted...@jp...>]

Inlined comments below...

At 12:41 PM 2/11/2005, John Hunter wrote:
> >>>>> "Ted" == Ted Drain <ted...@jp...> writes:
>
>     Ted> Is there an easy way to plot multiple collections of points
>     Ted> connected by a line and have only a single legend entry show
>     Ted> up?  It would also be nice if any modifications to that entry
>     Ted> (marker, color, etc) would affect all the line segments.
>
>Not 100% sure what you are after here.  You can control which lines
>and patches get passed to the legend by explicitly passing them (as
>opposed to using the autolegend capabilities).  Eg
>
>   ax.legend((l1, p1), ('A line, 'A patch')
>
>Also, have you looked into the LineCollection class -- this sounds
>like it would support some of what you are describing.  Perhaps if you
>explain a bit more.

What's happening here is that we have a plot that's a map of the earth 
going from -180 to 180 longitude and pole to pole.  A spacecraft ground 
trajectory goes around and around so when it hit's the +180 longitude, it 
needs to wrap around to -180.  We want this to be a line plot.  If you just 
put in the x,y coordinates as a line plot, you get a long line segment when 
it goes from say:
(175,20) -> (-175,30)

What we've done in the past is to use a heuristic to detect this case.  For 
example, I might say if the delta x is greater than 90% of the total 
longitude, it's a wrap around case.  In that case, I do linear 
interpolation to get a point at the edge of the map (in this case 180, 25), 
insert this point, and split the line into two pieces at this point.

For example, if I have a data set that looks like this:

[155,2]
[165,4]
[175,6]
[-175,8]
[-165,10]
[-155,12]

I really need this data to be drawn as two separate lines:
[155,2]
[165,4]
[175,6]
[180,7]

and
[-180,7]
[-175,8]
[-165,10]
[-155,12]

Ideally these two segments should be treated as a single line wherever 
applicable (legend, style, etc).




>     Ted> Ted PS: we also need some way to draw only the n'th marker in
>     Ted> a line plot.  We plot a lot of trajectories where time is
>     Ted> progressing along the line so it's useful to generate the
>     Ted> plot using 1 minute data (for example) and then have a marker
>     Ted> be displayed every 60'th point.
>
>Look at subplot(211) in examples/subplot_demo.py.  There a line is
>plotted with one temporal resolution, and markers are placed along the
>line at subsampled points.  Basically two lines are added with
>different sampling frequencies.
>
>Does this suffice?

I'd probably label this as a "mostly acceptable work-around" since it 
requires generating two data arrays.  In my case, the data is expensive to 
compute so we'd probably have to generate a second array by selectively 
copying points from the first array which is kind of annoying.  I was 
hoping for a keyword that said how often to generate the markers for an 
existing line (with the default as one of course).  Probably not a huge 
deal though.

>JDH

Ted Drain     Jet Propulsion Laboratory   ted...@jp...