PGPLOT Module: PGCONB -- contour map of a 2D data array, with blanking

SUBROUTINE PGCONB (A, IDIM, JDIM, I1, I2, J1, J2, C, NC, TR,
1 BLANK)
INTEGER IDIM, JDIM, I1, I2, J1, J2, NC
REAL A(IDIM,JDIM), C(*), TR(6), BLANK

Draw a contour map of an array. This routine is the same as PGCONS,
except that array elements that have the "magic value" defined by
argument BLANK are ignored, making gaps in the contour map. The
routine may be useful for data measured on most but not all of the
points of a grid.

Arguments
A (input) : data array.
IDIM (input) : first dimension of A.
JDIM (input) : second dimension of A.
I1,I2 (input) : range of first index to be contoured (inclusive).
J1,J2 (input) : range of second index to be contoured (inclusive).
C (input) : array of contour levels (in the same units as the
data in array A); dimension at least NC.
NC (input) : number of contour levels (less than or equal to
dimension of C). The absolute value of this
argument is used (for compatibility with PGCONT,
where the sign of NC is significant).
TR (input) : array defining a transformation between the I,J
grid of the array and the world coordinates. The
world coordinates of the array point A(I,J) are
given by:
X = TR(1) + TR(2)*I + TR(3)*J
Y = TR(4) + TR(5)*I + TR(6)*J
Usually TR(3) and TR(5) are zero - unless the
coordinate transformation involves a rotation
or shear.
BLANK (input) : elements of array A that are exactly equal to
this value are ignored (blanked).