Draw.io offers pre-built shapes, as well as raster and SVG images that you can pull in and use. This provides you with a lot of flexibility, but this approach has the downside that you can't style raster or SVG images. draw.io doesn't understand where to draw a shadow, which line(s) the line width applies to, etc.
Shapes can be edited by clicking the Edit shape button in the Style section of the Format Panel (Ctrl+Shift+P). Here is a description of the XML format. An instructional example that tries to demonstrate a number of elements and the XSD are also available.
The outer element is <shape>, that has attributes:
- "name", string, required. The stencil name that uniquely identifies the shape. Not used in draw.io currently.
- "w" and "h" are optional decimal view bounds. This defines your co-ordinate system for the graphics operations in the shape. The default is 100,100.
- "aspect", optional string. Either "variable", the default, or "fixed". Fixed means always render the shape with the aspect ratio defined by the ratio w/h. Variable causes the ratio to match that of the geometry of the current vertex.
- "strokewidth", optional string. Either an integer or the string "inherit". "inherit" indicates that the strokeWidth of the cell is only changed on scaling, not on resizing. If numeric values are used, the strokeWidth of the cell is changed on both scaling and resizing and the value defines the multiple that is applied to the width. Default is "1".
If you want to define specific fixed connection points on the shape use the <connections> element. Each <constraint> element within connections defines a fixed connection point on the shape. Constraints have attributes:
- "perimeter", required. 1 or 0. 0 sets the connection point where specified by x,y. 1 Causes the position of the connection point to be extrapolated from the center of the shape, through x,y to the point of intersection with the perimeter of the shape.
- "x" and "y" are the position of the fixed point relative to the bounds of the shape. They can be automatically adjusted if perimeter=1. So, (0,0) is top left, (0.5,0.5) the center, (1,0.5) the center of the right hand edge of the bounds, etc. Values may be less than 0 or greater than 1 to be positioned outside of the shape.
- "name", optional string. A unique identifier for the port on the shape.
<background> and <foreground>
The path of the graphics drawing is split into two elements, <foreground> and <background>. The split is to define which part any shadow applied to the shape is derived from (the background). This, generally, means the background is the line tracing of the outside of the shape, but not always.
Any stroke, fill or fillstroke of a background must be the first element of the foreground element, they must not be used within <background>. If the background is empty, this is not required.
Because the background cannot have any fill or stroke, it can contain only one <path>, <rect>, <roundrect> or <ellipse> element (or none). It can also not include <image>, <text> or <include-shape>.
Note that the state, styling and drawing in mxGraph stencils is very close in design to that of HTML 5 canvas. Tutorials on this subject, if you're not familiar with the topic, will give a good high-level introduction to the concepts used.
Rendering within the foreground and background elements has the concept of state. There are two types of operations other than state save/load, styling and drawing. The styling operations change the current state, so you can save the current state with <save/> and pull the last saved state from the state stack using <restore/>.
The elements that change colors within the current state all take a hash prefixed hex color code ("#FFEA80").
- <strokecolor>, this sets the color that drawing paths will be rendered in when a stroke or fillstroke command is issued.
- <fillcolor>, this sets the color that the inside of closed paths will be rendered in when a fill or fillstroke command is issued.
- <fontcolor>, this sets the color that fonts are rendered in when text is drawn.
<alpha> defines the degree of transparency used between 1.0 for fully opaque and 0.0 for fully transparent.
<strokewidth> defines the integer thickness of drawing elements rendered by stroking. Use fixed="1" to apply the value as-is, without scaling.
<dashed> is "1" for dashing enabled and "0" for disabled.
When <dashed> is enabled the current dash pattern, defined by <dashpattern>, is used on strokes. dashpattern is a sequence of space separated "on, off" lengths that define what distance to paint the stroke for, then what distance to paint nothing for, repeat... The default is "3 3". You could define a more complex pattern with "5 3 2 6", for example. Generally, it makes sense to have an even number of elements in the dashpattern, but that's not required.
<linejoin>, <linecap> and <miterlimit> are best explained by the Mozilla page on Canvas styling (about halfway down). The values are all the same except we use "flat" for linecap, instead of Canvas' "butt".
For font styling there are:
- <fontsize>, an integer,
- <fontstyle>, an ORed bit pattern of bold (1), italic (2) and underline (4), i.e bold underline is "5",
- <fontfamily>, is a string defining the typeface to be used.
Most drawing is contained within a <path> element. Again, the graphic primitives are very similar to that of HTML 5 canvas.
- <move> to attributes required decimals (x,y).
- <line> to attributes required decimals (x,y).
- <quad> to required decimals (x2,y2) via control point required decimals (x1,y1).
- <curve> to required decimals (x3,y3), via control points required decimals (x1,y1) and (x2,y2).
- <arc>, this doesn't follow the HTML Canvas signatures, instead it's a copy of the SVG arc command. The SVG specification documentation gives the best description of its behaviors. The attributes are named identically, they are decimals and all required.
- <close> ends the current subpath and causes an automatic straight line to be drawn from the current point to the initial point of the current subpath.
In addition to the graphics primitive operations there are non-primitive operations. These provide an easy method to draw some basic shapes:
- <rect>, attributes "x", "y", "w", "h", all required decimals
- <roundrect>, attributes "x", "y", "w", "h", all required decimals. Also "arcsize" an optional decimal attribute defining how large, the corner curves are.
- <ellipse>, attributes "x", "y", "w", "h", all required decimals.
Note that these 3 shapes and all paths must be followed by either a fill, stroke, or fillstroke.
<text> elements have the following attributes:
- "str", the text string to display, required.
- "x" and "y", the decimal location (x,y) of the text element, required.
- "align", the horizontal alignment of the text element, either "left", "center" or "right". Optional, default is "left".
- "valign", the vertical alignment of the text element, either "top", "middle" or "bottom". Optional, default is "top".
- "localized", 0 or 1, if 1 then the "str" actually contains a key to use to fetch the value out of mxResources. Optional, default is 0, unused in draw.io.
- "vertical", 0 or 1, if 1 the label is rendered vertically (rotated by 90 degrees). Optional, default is 0.
- "rotation", angle in degrees (0 to 360). The angle to rotate the text by. Optional, default is 0.
- "align-shape", 0 or 1, if 0 ignore the rotation of the shape when setting the text rotation. Optional, default is 1.
- "placeholders", 0 or 1, if 1 placeholders of the form %name% will be replaced with their values. Optional, default is 0.
<image> elements can either be external URLs, or data URIs, where supported (not in IE 7-). Attributes are:
- "src", required string. Either a data URI or URL.
- "x", "y", required decimals. The (x,y) position of the image.
- "w", "h", required decimals. The width and height of the image.
- "flipH" and "flipV", optional 0 or 1. Whether to flip the image along the horizontal/vertical axis. Default is 0 for both.
Sub-shapes (only supported for built-in shapes in draw.io)
<include-shape> allow stencils to be rendered within the current stencil by referencing the sub-stencil by name. Attributes are:
- "name", required string. The unique shape name of the stencil.
- "x", "y", "w", "h", required decimals. The (x,y) position of the sub-shape and its width and height.