How do graphics convey meaning? How do we perceive drawings?
In order to answer these questions, we need to understand how do we perceive the elements of drawings and spatial relation between these elements. In this essay, I am going to discuss lines as a graphical element and proximity as graphical space usage.
The basic structural component of drawings is line. It is an element of design that has length and direction. It is often used to communicate a specific feeling or draw viewers' attention to an important part in the design.  In the early 1970s, Arnheim proposed three ways of line usage - "lines as objects", "lines as edges" of objects and repeated "lines as texture". In the early 2000s, Massironi has added the forth way - "lines as a crack in a surface". 
- Object lines are extended in one direction only, do not divide space into parts and usually open-ended (are not connected to other lines). They are perceived to be "in front" of the background for viewers. Object lines can be useful to represent abstract geometric elements, virtual lines (such as trajectories, line of sight), and transformation of time in statistical graphs.
- Edge lines create contours and form. They are no longer objects, but edges of drawn objects. They usually form closed contours of figures and make the "inner" part of the space (enclosed region) appear to look "in front" of the background for the observers. We do not perceive them as lines, but as something that makes the figures stand out.
- Crack lines are defined as lines that separate two parts of a surface, or two juxtaposed objects. In other words, they are usually depicted inside of forms produced by edge lines. Examples of simple crack lines are eyes and mouth in an abstract drawing of a face.
- Texture lines form a texture, value and density for objects by repetition in regular or slightly regular way. As a rule, the elements of texture should be close to each other to stand out.
The way of lines depiction (spatial arrangement, directionality) defines the manner they are perceived and the meaning they communicate. In addition to cognitive functions stated above, lines can be used to create perspective, sense of continuance in a composition and a mood. 
Spatial relations of graphical components are used to convey relations based on proximity. Massironi has introduced four levels of proximity relations: nominal, ordinal, interval, and ratio. 
- Nominal relations use the simplest form of proximity - grouping which requires placing related things close to each other and unrelated things farther away. The concept of "proximity" in visual communication was introduced as one of the Gestalt principles of perceptual organization. It states that elements that are arranged close together are perceived as being relates.  In addition to spacing, other visual devices, like color, shading and texture are used. This goes along with another Gestalt principle - similarity, which is grouping visual elements by size, color, shape and style.  In other words, we perceive elements to be in the same group even if they are far apart, but have same features stated above.
- For ordinal relations, design elements that are used to indicate grouping are applied as well. An effect of order is provided by slaient usage of visual and spatial devises. For example visual objects successively spaced (indented), colored or sized.
- For interval relations the distances between elements are meaningful and for ratio relations, zero and the ratios of the distances between elements are important. For example, we observe values of distance in XY-plot and pie-charts. 
- W.D. Ellis. A Source Book of Gestalt Psychology. International library of psychology, philosophy and scienti?c method. Routledge, 1999.
- Forum.Nokia. Design and user experience library v2.1.
- M.Massironi. The psychology of graphic images: seeing, drawing, communicating. Volume in the University of Alberta, Department of Psychology, Distinguished Scholar Lecture. L. Erlbaum, 2002.
- Joshua David McClurg-Genevese. The elements of design, August 15, 2005.
- B. Tversky. Spatial Schemas and Abstract Thought, chapter Spatial Schemas in Depiction. Bradford Books. MIT Press, 2003.