What do graphs tell us

They do not show changes over time. Bar graphs are used to compare things between different groups or to track changes over time. However, when trying to measure change over time, bar graphs are best when the changes are larger. Area graphs are very similar to line graphs.

You might, for example, want to show how a budget had been spent on different items in a particular year. Line graphs show you how numbers have changed over time. They are used when you have data that are connected, and to show trends, for example, average night-time temperature in each month of the year.

Cartesian graphs have numbers on both axes, which therefore allow you to show how changes in one thing affect another. These are widely used in mathematics, and particularly in algebra. Graphs have two axes , the lines that run across the bottom and up the side. The line along the bottom is called the horizontal or x-axis , and the line up the side is called the vertical or y-axis. The numbers on the y-axis generally, but not always, start at 0 in the bottom left of the graph, and move upwards.

Usually the axes of a graph are labelled to indicate the type of data they show. Beware of graphs where the y-axis doesn't start at 0, as they may be trying to fool you about the data shown and there is more about this on our page, Everyday Mathematics.

Bar graphs generally have categories on the x-axis, and numbers on the y-axis but these are interchangeable. This means that you can compare numbers between different categories. The categories need to be independent, that is changes in one of them do not affect the others. You can see immediately that this graph gives you a clear picture of which category is largest and which is smallest. It gives a clear comparison between categories.

You can also use the graph to read off information about how many are in each category without having to refer back to the data table, which may or may not be provided with every graph you see. The bars do not touch. A histogram is a specific type of bar chart, where the categories are ranges of numbers. Histograms therefore show combined continuous data. Gantt charts illustrate project schedules.

The horizontal axis represents the timeframe for the project in days, weeks, months or years. The chart displays each project task as a bar on the vertical axis.

The length of the bar depends on the start and end date of the task, but sometimes there is also a vertical line for the current date. Project managers use Gantt charts to monitor the progress and completion status of each task. Waterfall charts reflect variance over time. They demonstrate both the positive and negative impact of different factors on an initial value, such as an opening balance.

Waterfall charts are helpful when illustrating financial statements, analyzing profit and loss and comparing earnings. You might use this chart to highlight the budget versus the amount spent. Positive and negative values usually follow a color code to show how the value increases or decreases due to a series of changes over time. Gauge charts display data as a reading on a dial. They show where a specific data point is within a minimum or maximum range. A needle depicts the value within a scale.

Many people use gauge charts to illustrate speed, revenue goals and temperatures. Funnel charts illustrate how values progress through different stages. They are widest at the top and narrowest at the bottom. Funnel charts are especially helpful when tracking a sales process.

They also work well to depict website traffic, including the number of visitors to a site, the pages viewed and downloads made. A bullet chart can help you measure the performance of a specific goal or target. Some bullet charts, such as those that demonstrate profits, have high targets. Others have low targets, including those that display expenses. People often use bullet charts in dashboards to illustrate the progress of key performance indicators KPIs.

A bullet chart is similar to a bar graph and consists of three parts:. When making decisions about students' interpretation of graphs, it is important to also consider their familiarity with the context. Lack of knowledge about the context may affect their ability to interpret the graph. This resource is intended for teachers to use for their own professional development.

Mathematics Complete and use a table to graph the cost per hour of repairing a car: Car maintenance. Science Complete a table about properties of paper towels: The best mopper upper. Science Interpret a graph of a car's journey and add to the graph to represent a further description of the journey: A car journey. Histograms Use histograms when y-axis gives the frequency of, or occurrences for continuous data that has been sorted into groups, for example, metres.

All bars are usually of equal width. They can be turned into line graphs by connecting the middle of the top section of each vertical bar. Histograms are not joined up bar graphs and should not be used for categoric data unless the number of units in each group is large. Identified numerical patterns generated from one or both axes. These were irrelevant to the graphs' "messages".

Some students were distracted from generalising about the relationships by obvious numerical patterns. Described the shape or direction of the line. These students did not relate this shape to what the axes represented. Described a general trend in the separate variables but did not relate these to one another, or described a trend in one variable but not the other. Were able to generalise a relationship between dependant and independent variables.

Prior experience of the context appeared to be a factor in being able to make generalisations. Questions Is the information presented appropriately for the design of the investigation? What does the table or graph not tell us about the design of the investigation? What does the information in the table or graph tell us?

Are there any patterns in the data? What does the data shown not tell us that might invalidate our interpretation? Do the patterns suggest an association, a difference, or a change between the variables? Can we use the pattern in the data to predict and generalise? This includes being aware of the limitations of the presentation of the data. Are there alternative interpretations for the pattern of the data?

Might other factors be causing the pattern?