Enable plotting spectra by sky position (from CAS-1814)

[Kana Sugimoto]

• enable plotting spectra based on their sky positions.
  
• users should be able to specify grid center, spacing, and the spatial extent over which the spectra should be sampled.
  

[Kana Sugimoto]

Date: 2012/06/26, 2012/07/17, 19
Rev.: 2576, 2600, 2602
Codes: asapplotter.py

Added a new method plotgrid in the asapplotter module.
asapplotter.plotgrid(scantable=None, center=None, spacing=None, rows=None, cols=None)

It plots the spectra in scantable based on sky position (stored in DIRECTION column).
This method doesn't grid spectra in a scantable. It just simply samples spectra in a scantable, based on parameter values specified by users, i.e., center, spacing, row, and cols. You may want to grid scantable using asapgrid module before plotting by this method.
Below is the descriptions of the parameters:

• center: a list of R.A. and Dec. which specifies the center of directions to be plotted in the direction frame and unit of DIRECTION column, e.g., [-1.309, 0.5236] (rad) for "J2000 19h00m00 30d00m00" (default: average direction if scantable.)
  
• spacing: a list of R.A. and Dec. spacing by which spectra are sampled in the unit of DIRECTION column, e.g, [0.000291,0.00291] (rad) for 1arcmin spacing. (default: extent of DIRECTION / number of rows or cols)
  
• rows & cols: the number of panels to plot (sample) in R.A. and Dec. (default: the value set by set_layout or if not set, rows = cols = 1)
  

NOTE: center and spacing should be specified in the original unit and direction frame of DIRECTION column in scantable.

Also note that stacking is not supported in this method. Currently only the first IFNO and POLNO in the scantable is plotted. You can plot the other IF and/or polarization by selecting the proper ID before plotting.

[Kana Sugimoto]

Since asapplotter.plotgrid neither grids scantable nor overlays spectra, only the first spectrum in the spacing is plotted on each panel of plot. This means when the value of spacing set is larger than the pointing spacing of observation of the scantable, only one spectrum in the spacing is sampled to plot and the other spectra are skipped.