While for normal situations accessing the Variable Star Plotter (VSP)  via the form is the way to go, if a program is being written that will display a chart, or someone wants to embed a specific chart on a web page, a chart can be created by creating a URL with chart parameters embedded in it. This is known on the web as the GET  method. This page documents how to do this for VSP.
The URL begins like so:
Then a string of variables is added to specify the desired chart and its desired appearance. For example, if a 2° chart for Z UMa with a limiting magnitude of 11.5 and a chart title of "Great Bear Variable" was wanted, the URL would look like:
http://www.aavso.org/cgi-bin/vsp.pl?name=Z UMa&charttitle=Great Bear Variable&fov=120&maglimit=11.5
That will give a chart on a blank web page. The following URL will give the same chart, but with the web site's header and footer included:
The "?" starts off the variable list, and a "&" separates the specific variables. Strings are not put in quotes.
Here are the variables that can be accessed.
name - The name of the star in question. Z UMa or U Gem are examples.
chartid - This variable can be used if you know the chart ID of a specific chart to regenerate.
ra - If a specific area of the sky is wanted, input its RA in the form HH:MM:SS.SS
dec - If a specific area of the sky is wanted, input its declination in the form +DD:MM:SS.S
ccdtable - Specifying ccdtable=on will generate a Field Photometry for your chart instead of a sky chart.
charttitle - This will be displayed at the top of the chart page. Remember strings don't have quote marks.
chartcomment - These will appear at the bottom of the chart.
dss - Specifying dss=on will load a Digital Sky Survey photograph instead of plotting stars on a white background.
forcelines - Specifying forcelines=on will force lines to be drawn from all magnitude labels to their corresponding stars.
fov - Field of view, expressed in minutes of arc.
maglimit - This variable will specify the faintest magnitude visible on the chart.
othervars - If gcvs is specified the chart will show all GCVS variables in your chosen field. If all is specified the chart will show all known variables in the field.
printable - You can specify yes or no with this variable. If no (the default) is specified, the chart will have a screen resolution of 75dpi. If yes is specified, it will have a resolution of 150dpi and the header and footer HTML code will not be displayed.
delimited - this will publish a list of the comp star photometry in a tabular forma t.
north - This can be set to up or down. The northern edge of the field will be oriented in that direction.
east - This can be set to right or left. The eastern edge of the field will be oriented in that direction.
Are any of these variables absolutely needed as a minimum? No one specific variable is needed, but at a minimum a chart must have a star name specified, or an RA and Dec, or a chartid. All the other variables listed above are modifiers.