Here are some broad guidelines for Graphs from EIA.gov , so you can say these are the official graphical guidelines of USA Gov
They can be really useful for sites planning to get into the Tableau Software/NYT /Guardian Infographic mode- or even for communities of blogs that have recurrent needs to display graphical plots- particularly since communication, statistical and design specialists are different areas/expertise/people.
Energy Information Administration Standard
Energy Information Administration Standard 2009-25
Title: Statistical Graphs
Superseded Version: Standard 2002-25
Purpose: To ensure the utility (usefulness to intended users) and objectivity (accuracy, clarity, completeness, and lack of bias) of energy information presented in statistical graphs.
Applicability: All EIA information products.
- Graphs should be used to show and compare changes, trends and/or relationships, and to assist users in visualizing the conclusions drawn from the data represented.
- A graph should contain sufficient information to either be understood by itself or be consistent with the written text When possible, information used to interpret the graph should either be visible from the web page without scrolling, accessible from the web page through a link, or on the same page in the printed product where the graph appears.
- The source of the data used for the graph should be shown at the bottom of each graph and contain a link to the web page where the data may be accessed.
- Graph titles and axis labels should be clear and descriptive with no unexplained or undefined acronyms or industry jargon. If acronyms are used because of space limitations, they must comply with EIA Standard 16 “Codes, Abbreviations, Acronyms, and Definitions.
- Both axes of a graph should be labeled with the names of variables, except where the x-axis label “years” is obvious. The vertical axis should show the units of measurement unless that information is available from the title, legend or other area of the graph. The vertical axis should start with either zero or an appropriate minimum value of the scale that does not distort the relationships between data series.
- When using time intervals, spacing should be equidistant only if the intervals are equidistant.
- For compliance with accessibility guidelines (Section 508), graphs must be clear and understandable when printed or viewed in black and white. Web graphs must contain alternative “ALT” text tags that describe and summarize the graph for use by screen readers.
- Standard 2009-25 Supplementary Materials, Guidelines for Graphs
Types of Graphs
- A bar graph is used to show relationships between groups. The two or more data series being compared do not need to affect each other.
- A line graph is used to show continuous data series and/or how one data series is affected by another.
- A circle or pie graph is used to show how a part of something relates to the whole.
- Use a title that summarizes the key point or message of the graph or what the data represent, the geographic area and time period represented by the data.
- Place the title flush left or centered.
Layout and Scale
- Avoid clutter that does not add necessary information for interpreting the graph (e.g., too many arrows, bubble boxes, grid lines, extra tick marks, and other non-data features).
- When possible, order values from highest to lowest or from lowest to highest if the data are not a time series.
- Use the same scale whenever appropriate, such as when a series of graphs is related by measuring the same product price or unit of supply across different geographic areas, so that the related graphs can be compared using a common scale.
Labels, Legends, and Lines
- Avoid using legends when the data series can be identified from information in the graph such as the title or data series labels.
- Use labels on bars, lines or pie slices whenever possible for identifying different data series.
- Use simple drawings, symbols, or cartoon images to depict quantities in a pictogram.
- Do not use large symbols or patterns to draw lines.
Dual axis graphs
- Labeling is important when using a dual axis graph. Different scales are permitted on a dual axis graph.
- Different chart types may be used for the variables, such as bars for volumes and lines for prices.
- Use different colors for each variable and associate the variable’s color with the axis label to help users determine which y-axis to use. For example, graph the price with a red line and show the price y-axis in red and graph the volume line in blue and show the volume y-axis in blue.
- Stacked bar graphs and cumulative line graphs are not permitted in dual axis graphs.
Cumulative (stacked) graphs
- Stacked graphs are primarily used when the focus is on the component segments and their relation to the total value and the components do not exhibit seasonality, marked irregularities, or sharp upward or downward trends.
- With stacked graphs, users tend to add the various bar segments when trying to estimate the value, rather than visualizing the trends of the stacked bars separately. Show trends with clustered (side-by-side) bar charts rather than with stacked bars.
- Avoid using cumulative (stacked) line graphs or stacked bar graphs for time series data unless it is necessary to show changes in the component segments over time as a key point in the graph. Include the data values or percentages for each bar segment when using cumulative bar graphs whenever possible.
- Avoid using three dimensional graphs when a two dimensional graph will present the same information.
- The data for the graph should be available in a corresponding table, whenever possible. For example, a graph on EIA’s website may have the data imbedded in the graph or accessible through a link at the bottom of the graph.
- The underlying data should contain statistical aggregates or source data that do not require protection.
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