Circos > Documentation > Tutorials > Recipes > Complex Histograms
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# 8 — Recipes

## 6. Manipulating Histograms

### mapping value to histogram fill

The fill of histogram bins can be changed based on the histogram bin value using a rule like the following.

```# remap the histogram value from the range [-0.6,0.6] onto the index [0,5], then
# use the index to select the color from the list
# dorange orange lorange lblue blue dblue
#
# Perl syntax for referencing an element of a list is qw(a b c d)[index]
# where index starts at 0.
<rule>
condition  = 1
fill_color = eval(qw(dorange orange lorange lblue blue dblue)[remap_int(var(value),-0.6,0.6,0,5)])
</rule>
```

### layered background color and macros

Recall that you can use `conf()` to reference a configuration parameter. This is described in detail in the Automation and Macros tutorial.

In this example, I create a background for the histogram track that has six colored layers: tones of blue for positive values and tones of orange for negative values. Both cutoffs for the layers and colors will be defined as parameters within the <plot> block.

Each separate colored layer in the background is defined by its own <background> block.

```<plot>
...
# define background color cutoffs
bgy1 = 0.2    # first cutoff
bgy2 = 0.5    # second cutoff
bgc1 = orange # bg color for negative values
bgc2 = blue   # bg color for positive values
...

<backgrounds>
# use the parameters above using conf(.,param)
<background>
color = lconf(.,bgc2)
y0    = conf(.,bgy2)
</background>
<background>
color = vlconf(.,bgc2)
y1    = conf(.,bgy2)
y0    = conf(.,bgy1)
</background>
<background>
color = vvlconf(.,bgc2)
y1    = conf(.,bgy1)
y0    = 0
</background>
<background>
color = vvlconf(.,bgc1)
y1    = 0
y0    = -conf(.,bgy1)
</background>
<background>
color = vlconf(.,bgc1)
y1    = -conf(.,bgy1)
y0    = -conf(.,bgy2)
</background>
<background>
color = lconf(.,bgc1)
y1    = -conf(.,bgy2)
</background>
</backgrounds>

</plot>
```

### y-axis grid and macros

The axis grid can also reference the histogram cutoff values.

```
<axes>

# every 25% with low transparency
<axis>
color     = grey_a1
thickness = 2
spacing   = 0.25r

# every 5% with higher transparency
<axis>
color     = grey_a3
thickness = 1
spacing   = 0.05r
</axis>

# thick line at y=0
<axis>
color     = grey_a1
thickness = 5
position  = 0
</axis>

# at each background cutoff, a thick white line
<axis>
color     = white
thickness = 5
position  = -conf(.,bgy2),-conf(.,bgy1),conf(.,bgy1),conf(.,bgy2)
</axis>

</axes>
```