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3.15.2.8 keywords
The following keywords apply only when generating coordinates from binary data files. That is, the control mapping the individual elements of a binary array, matrix, or image to specific x/y/z positions.
— SCAN —
A great deal of confusion can arise concerning the relationship between how gnuplot scans a binary file and the dimensions seen on the plot. To lessen the confusion, conceptually think of gnuplot _always_ scanning the binary file point/line/plane or fast/medium/slow. Then this keyword is used to tell gnuplot how to map this scanning convention to the Cartesian convention shown in plots, i.e., x/y/z. The qualifier for scan is a two or three letter code representing where point is assigned (first letter), line is assigned (second letter), and plane is assigned (third letter). For example, ‘scan=yx‘ means the fastest, point-by-point, increment should be mapped along the Cartesian y dimension and the middle, line-by-line, increment should be mapped along the x dimension.
When the plotting mode is ‘plot‘, the qualifier code can include the two letters x and y. For ‘splot‘, it can include the three letters x, y and z.
There is nothing restricting the inherent mapping from point/line/plane to apply only to Cartesian coordinates. For this reason there are cylindrical coordinate synonyms for the qualifier codes where t (theta), r and z are analogous to the x, y and z of Cartesian coordinates.
— TRANSPOSE —
Shorthand notation for ‘scan=yx‘ or ‘scan=yxz‘.
— DX, DY, DZ —
When gnuplot generates coordinates, it uses the spacing described by these keywords. For example ‘dx=10 dy=20‘ would mean space samples along the x dimension by 10 and space samples along the y dimension by 20. ‘dy‘ cannot appear if ‘dx‘ does not appear. Similarly, ‘dz‘ cannot appear if ‘dy‘ does not appear. If the underlying dimensions are greater than the keywords specified, the spacing of the highest dimension given is extended to the other dimensions. For example, if an image is being read from a file and only ‘dx=3.5‘ is given gnuplot uses a delta x and delta y of 3.5.
The following keywords also apply only when generating coordinates. However they may also be used with matrix binary files.
— FLIPX, FLIPY, FLIPZ —
Sometimes the scanning directions in a binary datafile are not consistent with that assumed by gnuplot. These keywords can flip the scanning direction along dimensions x, y, z.
— ORIGIN —
When gnuplot generates coordinates based upon transposition and flip, it attempts to always position the lower left point in the array at the origin, i.e., the data lies in the first quadrant of a Cartesian system after transpose and flip.
To position the array somewhere else on the graph, the origin keyword directs gnuplot to position the lower left point of the array at a point specified by a tuple. The tuple should be a double for ‘plot‘ and a triple for ‘splot‘. For example, ‘origin=(100,100):(100,200)‘ is for two records in the file and intended for plotting in two dimensions. A second example, ‘origin=(0,0,3.5)‘, is for plotting in three dimensions.
— CENTER —
Similar to origin, this keyword will position the array such that its center lies at the point given by the tuple. For example, ‘center=(0,0)‘. Center does not apply when the size of the array is ‘Inf‘.
— ROTATE —
The transpose and flip commands provide some flexibility in generating and orienting coordinates. However, for full degrees of freedom, it is possible to apply a rotational vector described by a rotational angle in two dimensions.
The ‘rotate‘ keyword applies to the two-dimensional plane, whether it be ‘plot‘ or ‘splot‘. The rotation is done with respect to the positive angle of the Cartesian plane.
The angle can be expressed in radians, radians as a multiple of pi, or degrees. For example, ‘rotate=1.5708‘, ‘rotate=0.5pi‘ and ‘rotate=90deg‘ are equivalent.
If origin is specified, the rotation is done about the lower left sample point before translation. Otherwise, the rotation is done about the array ‘center‘.
— PERPENDICULAR —
For ‘splot‘, the concept of a rotational vector is implemented by a triple representing the vector to be oriented normal to the two-dimensional x-y plane. Naturally, the default is (0,0,1). Thus specifying both rotate and perpendicular together can orient data myriad ways in three-space.
The two-dimensional rotation is done first, followed by the three-dimensional rotation. That is, if R’ is the rotational 2 x 2 matrix described by an angle, and P is the 3 x 3 matrix projecting (0,0,1) to (xp,yp,zp), let R be constructed from R’ at the upper left sub-matrix, 1 at element 3,3 and zeros elsewhere. Then the matrix formula for translating data is v’ = P R v, where v is the 3 x 1 vector of data extracted from the data file. In cases where the data of the file is inherently not three-dimensional, logical rules are used to place the data in three-space. (E.g., usually setting the z-dimension value to zero and placing 2D data in the x-y plane.)
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