Gallery: Histograms

Examples

Displaying (x,y) data points as a histogram
Displaying (xlow,xhigh,y) data points as a histogram
A histogram which shows the bin edges
A histogram showing the full range of options
Filling a histogram with a pattern

1) Displaying (x,y) data points as a histogram

Histograms are used to plot binned one-dimensional data - of the form (x_mid, y) or (x_low, x_high, y) - with optional error bars on the Y values. A later example shows how to display error bars on the points.

Version: Postscript; PDF

add_histogram("spectrum.fits[fit][cols x,y]")

In this example the data is stored as a set of (x, y) points, where the x values give the center of each bin. Unlike curves, histograms default to only being drawn by a solid line; the symbol.style attribute is set to none.

The preferences for curves can be found by using the get_preference call:

chips> get_preference("histogram")

histogram.stem         : hist
histogram.depth        : default
histogram.line.color   : default
histogram.line.thickness: 1
histogram.line.style   : solid 
histogram.symbol.color : default
histogram.symbol.style : none
histogram.symbol.size  : 5
histogram.symbol.angle : 0
histogram.symbol.fill  : false
histogram.err.color    : default
histogram.err.thickness: 1
histogram.err.style    : line 
histogram.err.up       : on
histogram.err.down     : on
histogram.err.caplength: 10
histogram.dropline     : off
histogram.fill.color   : default
histogram.fill.opacity : 1
histogram.fill.style   : nofill

The settings for the current histogram can be found by using the get_histogram routine:

chips> get_histogram()

depth = 100
dropline = False
err.caplength = 10
err.color = default
err.down = False
err.style = line
err.thickness = 1.0
err.up = False
fill.color = default
fill.opacity = 1.0
fill.style = 0
id = None
line.color = default
line.style = 1
line.thickness = 1.0
stem = None
symbol.angle = 0.0
symbol.color = default
symbol.fill = False
symbol.size = 5
symbol.style = 0

2) Displaying (xlow,xhigh,y) data points as a histogram

In the first example, the binned data was given using the mid-point of each bin. In this example we show how histograms can be plotted by giving the low and high edges of each bin.

Version: Postscript; PDF

tbl = read_file("spectrum.fits[fit]")
xlo = copy_colvals(tbl,"xlo")
xhi = copy_colvals(tbl,"xhi")
y = copy_colvals(tbl,"y")
add_histogram(xlo,xhi,y,["line.color","red"])
log_scale(X_AXIS)

The bin edges can not be specified by passing in a file name to add_histogram, so we have to read in the arrays using crates routines and then plot them. We use the read_file to read in the file, copy_colvals to get the column values, and then add_histogram to plot them.

3) A histogram which shows the bin edges

When using x_mid values, the bins are assumed to be contiguous. This is not the case when the bin edges - namely x_low and x_high - are given. Here we plot data from a histogram with non-contiguous bins, setting the dropline attribute so that all the bin edges are drawn (this attribute can also be used with histograms like that used in the first example).

Version: Postscript; PDF

tbl = read_file("histogram.fits")
xlo = copy_colvals(tbl,"xlo")
xhi = copy_colvals(tbl,"xhi")
y = copy_colvals(tbl,"y")
add_histogram(xlo,xhi,y,["line.style","longdash","dropline",True])

4) A histogram showing the full range of options

In this example we change most of the attributes of a histogram. The Filling a histogram with a pattern example shows how you can change the fill style of a histogram from solid to a pattern.

Version: Postscript; PDF

tbl = read_file("histogram.fits")
xlo = copy_colvals(tbl,"xlo")
xhi = copy_colvals(tbl,"xhi")
y = copy_colvals(tbl,"y")
dylo = copy_colvals(tbl,"dylo")
dyhi = copy_colvals(tbl,"dyhi")
hist = ChipsHistogram()
hist.dropline = True
hist.line.color = "red"
hist.symbol.style = "diamond"
hist.symbol.size = 4
hist.symbol.fill = True
hist.symbol.color = "orange"
hist.err.color = "green"
hist.fill.style = "solid"
hist.fill.opacity = 0.2
hist.fill.color = "blue"
add_histogram(xlo,xhi,y,dylo,dyhi,hist)

# Move the histogram behind the axes so that the tick marks are not hidden
shuffle_back(chips_histogram)

A note on filling histograms

There are two issues when using the solid fill pattern:

Opacity values are not used in the postscript output, in that they are always taken to be 1.
The off-screen output may show a pattern of lines within the filled region for postscript and PDF outputs, depending on what display program you are using. They should not appear when printed out.

5) Filling a histogram with a pattern

In this example we fill the histogram using a pattern, rather than a solid fill as used in the A histogram showing the full range of options example.

Version: Postscript; PDF

add_histogram("spectrum.fits[fit][cols x,y]")
set_histogram(["fill.style","crisscross"])
set_histogram(["fill.color","green","line.color","red"])

The fill.style attribute of histograms is used to determine how the region is filled. Here we use the value "crisscross", rather than "solid", to fill the histogram with crossed lines. These lines can be colored independently of the histogram boundary.