Question:

How can I do overlays on a plot with a formatted time axis?

Explanation:

To overlay symbols or mark-up on a plot which has a formatted time axis (dates and times) it is necessary to specify positions using the internal time encoding of that axis. Typically, the easiest way to achieve this is to define a variable, say TT, which is the time encoding. This example illustrates. These examples are taken from the Ferret Users Guide, Chapter 6.

Example:

demonstrate PLOT/VS and POLYGON over time plot

The script that produces the above plot:

use coads_climatology
LET xsqr = {-1,1,1,-1} ! coordinates of a unit square
LET ysqr = {-1,-1,1,1}

LET xcircle = COS(6.3*i[i=1:42]/40) ! coordinates of unit circle
LET ycircle = SIN(6.3*i[i=1:42]/40)

! Notice the units of the time axis
show grid/l=1:3 sst

plot/x=180/y=0 sst ! draw a time series plot

let tt = T[gt=sst] ! tt is the coordinates along the T axis

! place an "X" at the value exactly at 7-aug
! "@ITP" causes interpolation to exact location

let t0 = tt[t="7-aug-0000"@itp]
let val0 = sst[x=180,y=0,t="7-aug-0000"@itp]
plot/vs/over/nolab/line=8/color=red t0,val0

! put a box around the "X"
polygon/over/line=8/title="Special region" t0+500*xsqr, 0.05*ysqr+val0

! place an "X" on the data point nearest to 15-may
! Note that @ITP is absent, so behavior is set by MODE INTERPOLATE

let t1 = tt[t="15-may-0000"]
let val1 = sst[x=180,y=0,t="15-may-0000"]
plot/vs/over/nolab/sym=2/line=10 t1,val1

! put a circle around the "X"
plot/vs/over/line=10/nolab t1+500*xcircle,0.05*ycircle+val1

Example (continued):

mark-up over a Hovmoller (time vs space) diagram

And here is the script:

use coads_climatology
let tt = T[gt=sst] ! tt is the coordinates along the T axis

shade/x=180 sst ! latitude vs time plot
let tlo = tt[t="1-jul-0000"@itp]
let thi = tt[t="1-aug-0000"@itp]

polygon/over/line=7/pal=gray/pat=lite_up_left_to_right {`tlo`, `thi`,` thi`, `tlo`}, {20, 20, 40, 40}