Add ability to read chart size from an attribute

[dmorissette]

During the questions/comments period of RFC-29 (Dynamic Charting Capability, see also ticket #1800), one of the questions raised was whether we could provide the ability to bind the chart size to an attribute.

This would be a nice to have but will not be included in the initial RFC implementation. Instead I propose that we keep this as a future enhancement. Here is the approach that I proposed on the mapserver-dev list:

I've thought about this some more. It is probably not sufficient to provide the ability to lookup the chart size in an attribute, since this attribute may not be a value in pixels but would be an actual value (e.g. total amount of sales) so you really need to be able to define a range of sizes for the chart which is driven by an attribute.

We could be to keep PROCESSING CHART_SIZE as is, and define a second mechanism to specify the chart size:

 PROCESSING "CHART_SIZE_RANGE" "itemname minsize maxsize minval maxval"

where:

• itemname is the name of the attribute that drives the chart size (e.g. total_sales)

• minsize and maxsize are the minimum and maximum chart size values in pixels (e.g. "10 100")

• minval and maxval are the minimum values of the attribute that correspond to chart sizes of minsize and maxsize (e.g. 10000 1000000).

If the attribute value is smaller than 'minval' then the chart size will be minsize pixels, and if the attribute value is larger than maxval, the chart size will be maxsize pixels.