National Oceanic and
Atmospheric Administration
United States Department of Commerce

labeling dates along a trackline

labeling dates along a trackline

Question:

How can I label the dates along a track plot?

Example:

 

a map plot with time labels along a track-line

map_time_labels.png, by ansley

Solution:

Use the TAX_datestring function to get date strings for a subset of the time steps.

 

! Define a dummy track line, position as a function of time.

DEFINE AXIS/T=1-JAN-1980:31-JAN-1985:30/UNIT=days tax 
LET ypos = 40 *COS(L[GT=tax]/32) 
LET xpos = 220 - 50 *SIN(L[GT=tax]/22) 

! Make the basic track plot

GO basemap X=110E:40W Y=20S:50N 20 
PLOT/VS/NOLAB/OVER XPOS,YPOS 

! Label every 5th date. The labels could be positioned differently, using
! the 3rd argument of the LABEL command to control centering. Or, to put the
! labels a bit above the corresponding point location, add a small amount to
! the y positions, for instance `ypos + 0.04` in the second argument to LABEL.

LET tt = t[gt=tax] 

! Loop over L; at each L we evaluate TAX_DATESTRING for the time step,
! and compute the location for the label

REPEAT/L=5:60:5 (DEFINE SYMBOL tlab = "`TAX_DATESTRING(tt,tt,"day")`"; \
 LABEL `xpos` `ypos` 1 0 0.1 ($tlab) )

 

A further example:

 

 

Here is an alternative way to get the time labels. This does not give as much control over the date string, however. We can use the timeaxis and the `RETURN=tstart` syntax:

 

!... Set up the variables as in the above example

REPEAT/L=5:60:5 (DEFINE SYMBOL tlab = "`T[GT=tax],RETURN=tstart`"; \
 LABEL `xpos` `ypos` 1 0 0.1 ($tlab) )