[ncl-talk] Drawing several polygons on a xy plot

[Dave Allured - NOAA Affiliate]

P.S.  I just remembered you are making a panel plot.  You will probably
need to increase the size of the graphics array to hold all polygons in all
six panels, due to that memory requirement.  Something like this:

    poly = new ( (/ nyears, npanels /), graphic)

There is more than one way to do this.  The array can be 1-D, 2-D, or 3-D,
whatever you think is most convenient for the looping and indexing
structure of your program.


On Wed, Mar 10, 2021 at 5:54 AM Dave Allured - NOAA Affiliate <
dave.allured at noaa.gov> wrote:

> Gsn_add_polygon does not work like that.  It draws only a single polygon
> per call, and you must specify all vertices of the polygon.  Missing values
> can not be used to create discontinuities or unfilled areas of a polygon.
>
> The simplest approach is to draw multiple polygons, one narrow rectangle
> for each year on the X axis.  Loop over years, and skip (i.e. do not draw)
> polygons unless the IPO index is negative.  Within the loop, remember to
> assign each polygon to a different element of a type graphics array, to
> satisfy the memory requirement of the function.
>
>     poly = new (nyear, graphic)
>     do i = 0, nyear-1
>         if (temp_amip2_tpi_year_filter(i) .lt. 0) then
>             poly(i) = gsn_add_polygon (...)
>         end if
>     end do
>
> I leave it up to you to calculate the correct polygon vertices for each
> year.  Your initial code version is almost doing this already.  I recommend
> using a fully closed polygon for each year.  That is 5 vertices, not 4, for
> each rectangle.  Function behavior for non-closed polygons is undefined.
>
>
> On Wed, Mar 10, 2021 at 3:20 AM Giorgio Graffino via ncl-talk <
> ncl-talk at mailman.ucar.edu> wrote:
>
>> Short addition to this thread: I'm running NCL 6.6.2 on Linux
>> 5.4.101-1-MANJARO.
>>
>> Best,
>> Giorgio
>>
>> ----Messaggio originale----
>> Da: ncl-talk at mailman.ucar.edu
>> Data: 10-mar-2021 0.11
>> A: <ncl-talk at ucar.edu>
>> Ogg: [ncl-talk] Drawing several polygons on a xy plot
>>
>> Dear NCL people,
>>
>> I'm trying to draw several polygons on a time series plot, to obtain
>> something like this (
>> https://www.nature.com/articles/nclimate2106/figures/1). Basically, I
>> need separate filled boxes to be drawn anywhere the IPO index is negative.
>> I'd like to not draw individual polygons myself, because I should set the x
>> and y ranges explicitly. (That is, at all times when the IPO index changes
>> sign). Rather, I'd like the boxes to be drawn automatically, without
>> knowing where the index is negative.
>>
>> So far, my best attempt at the problem is to create a 1-D mask, setting
>> all positive value of the index to missing values. Then, I'm creating one
>> big box over the whole domain, relying on the missing values of the masked
>> time series. The relevant snippet of my code is shown here.
>>
>> ;; Create polygons where AMIP II TPI/IPO index is negative
>>  temp_amip2_tpi_year_filter_neg =
>> where(temp_amip2_tpi_year_filter.lt.0.,temp_amip2_tpi_year_filter,temp_amip2_tpi_year_filter@
>> _FillValue)
>>  temp_amip2_tpi_year_filter_neg_xp =
>> new((/2*nyear/),typeof(temp_amip2_tpi_year_filter_neg),temp_amip2_tpi_year_filter_neg@
>> _FillValue)
>>  temp_amip2_tpi_year_filter_neg_yp =
>> new((/2*nyear/),typeof(temp_amip2_tpi_year_filter_neg),temp_amip2_tpi_year_filter_neg@
>> _FillValue)
>> do i = 0, nyear-1
>>  temp_amip2_tpi_year_filter_neg_yp(i)          =
>> temp_amip2_tpi_year_filter_neg(i)+1.e2
>>  temp_amip2_tpi_year_filter_neg_xp(i)          = years(i)
>>  temp_amip2_tpi_year_filter_neg_xp(2*nyear-1-i) = years(i)
>>  temp_amip2_tpi_year_filter_neg_yp(2*nyear-1-i) =
>> temp_amip2_tpi_year_filter_neg(i)-1.e2
>> end do
>>
>> After, I'm calling gsn_add_polygon to draw the boxes over the time series
>> plot, but I'm getting only one big box (see attached). It seems that the
>> function doesn't care about missing values, and in the description that's
>> clearly stated ("If a missing value is encountered in *x* and/or *y*,
>> then this pair is ignored, but the polygon will still be closed";
>> https://www.ncl.ucar.edu/Document/Graphics/Interfaces/gsn_add_polygon.shtml).
>>
>>
>> My question: is there a way to make gsn_add_polygon not closing the
>> polygon? Is there a better way to draw those polygons with NCL?
>>
>> Best,
>> Giorgio
>>
>>
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