httpgd API

httpgd can be accessed both from R and from HTTP/WebSockets.

Overview

R	HTTP	Description
hgd()		Initialize device and start server.
hgd_close()		Helper: Close device.
hgd_url()		Helper: URL generation.
hgd_browse()		Helper: Open browser.
ugd_state()	/state	Get current server state.
ugd_renderers()	/renderers	Get list of available renderers.
ugd_render()	/plot	Get rendered plot (any format).
ugd_clear()	/clear	Remove all plots.
ugd_remove()	/remove	Remove a single plot.
ugd_id()	/plots	Get static plot IDs.
	/live	Live server page.

Get state

While all the APIs can be accessed stateless, the graphics device does have a state defined by.

Field	Type	Description
upid	int	Update id. Changes when plots are removed or when something is drawn.
hsize	int	Number of plots in the history.
active	bool	Whether the graphics device is active. When another graphics device is activated, the device will become inactive and not be able to render any plots that are not cached (no resizes).

To receive state changes as they happen WebSockets can be used. Alternatively /state may be polled repeatedly.

From R

unigd::ugd_state()

Note: Prior to httpgd 2.0 this function also returned host, port and security token of the server. These fields are now accessed via hgd_details().

From HTTP

/state

Key	Value	Default
token	Security token.	(The X-HTTPGD-TOKEN header can be set alternatively.)

Will respond with a JSON object.

From WebSockets

httpgd accepts WebSocket connections on the same port as the HTTP server. Server state changes will be broadcasted immediately to all connected clients in JSON format.

Get Renderers

httpgd includes multiple renderers that can dynamically render plots to different target formats. As new formats may be added as the development on httpgd continues, and some depend on optional system dependencies, a list of available renderers can be obtained during runtime.

The following is a complete list of renderers.

ID	Mime-Type	Renderer	Format
eps	application/postscript	Encapsulated PostScript (EPS).	Text
json	application/json	Plot data serialized to JSON format.	Text
meta	application/json	Plot meta information.	Text
pdf	application/pdf	Adobe Portable Document Format (PDF).	Binary
png	image/png	Portable Network Graphics (PNG).	Binary
png-base64	text/plain	Base64 encoded Portable Network Graphics (PNG).	Text
ps	application/postscript	PostScript (PS).	Text
strings	text/plain	List of strings contained in plot.	Text
svg	image/svg+xml	Scalable Vector Graphics (SVG).	Text
svgp	image/svg+xml	Version of the SVG renderer that produces portable SVGs.	Text
svgz	image/svg+xml	Compressed Scalable Vector Graphics (SVGZ).	Binary
svgzp	image/svg+xml	Version of the SVG renderer that produces portable SVGZs.	Binary
tikz	text/plain	LaTeX TikZ code.	Text

From R

unigd::ugd_renderers()

Returns a data frame.

From HTTP

/renderers

Key	Value	Default
token	Security token.	(The X-HTTPGD-TOKEN header can be set alternatively.)

Render plot

Plots can be rendered in various file formats from both R and HTTP. The actual plot construction in R is relatively slow so httpgd caches the plot in the last requested size. Subsequent calls with the same width and height or without a size specified will always be fast. (This way “flipping” through plot pages is very fast.)

From R

Example:

unigd::ugd_render(page = 3, width = 800, height = 600) # Get plot at index 3 with 800*600
unigd::ugd_render() # Get last plot with cached size

page can either be a number to indicate a plot index or a static plot ID (see: hgd_id()).

This function returns the plot as a string. The file attribute can be used to save the SVG directly to disk.

From HTTP

Example:

/plot?index=2&width=800&height=600

Parameters:

Key	Value	Default
width	With in pixels.	Last rendered width. (Initially device width.)
height	Height in pixels.	Last rendered height. (Initially device height.)
zoom	Zoom level.	1 (No zoom). 0.5 would be 50% and 2 200%.
index	Plot history index.	Newest plot.
id	Static plot ID.	index will be used.
renderer	Renderer.	svg.
token	Security token.	(The X-HTTPGD-TOKEN header can be set alternatively.)

Note that the HTTP API uses 0-based indexing and the R API 1-based indexing. This is done to conform to R and JavaScript on both ends. (This means the the first plot is accessed with /plot?index=0 and unigd::ugd_render(page = 1).)

Remove plots

From R

Examples:

unigd::ugd_remove(page = 2) # Remove the second page
unigd::ugd_clear() # Clear all pages

From HTTP

Examples:

/remove?index=2
/clear

Key	Value	Default
index	Plot history index.	Newest plot.
id	Static plot ID.	index will be used.
token	Security token.	(The X-HTTPGD-TOKEN header can be set alternatively.)

Get static IDs

The problem with requesting individual plots by index is, that a plots index will change when earlier plots are removed from the plot history. To circumvent this, each plot also is assigned a static ID.

All APIs that access individual plots can also be called with static IDs instead of indices.

From R

Examples:

unigd::ugd_id(index = 2) # Static ID of the second plot
unigd::ugd_id() # Static ID of the last plot

Note: The limit parameter can be adjusted to obtain multiple or all plot IDs.

From HTTP

Examples:

/plots?index=2
/plots

Key	Value	Default
index	Plot history index.	Newest plot.
limit	Number of subsequent plot IDs.	1
token	Security token.	(The X-HTTPGD-TOKEN header can be set alternatively.)

Notes:

• The limit parameter can be specified to support pagination.
• The JSON response will contain the state to allow checking for desynchronisation.

Security

A security token can be set when starting the device:

hgd(..., token = "secret")

When set, each API request has to include this token inside the header X-HTTPGD-TOKEN or as a query param ?token=secret. token is by default set to TRUE to generate a random 8 character alphanumeric token. If it is set to a number, a random token of that length will be generated. FALSE deactivates the security token.

CORS is off by default but can be enabled on startup:

hgd(..., cors = TRUE)