Friday, 2 September 2011

Bubbles - The Chart-ed Path

Knowing Bubble Graphs
What you do when you have a table with 3 related dimensions of data, how do you represent it on a flat 2D chart? The answer to this question is Bubble chart. Bubble chart helps us in putting 3 parameters in one elegant visualization..
A bubble chart is a type of chart where each plotted entity is defined in terms of three distinct numeric parameters.
The entities displayed on a bubble chart can be compared in terms of their size as well as their relative positions with respect to each numeric axis. Since both X and Y axis of the bubble chart is numeric scales, the position of plot is an indicator of two distinct numeric values. The area of the plot depends on the magnitude of a third numeric characteristic. One concern when rendering data with a bubble chart is that the area of a circle is proportional to the square of the radius. So if you scale the radius with your third data point, you will disproportionally emphasize the third factor. To get a properly weighted scale, one should take the square root of the magnitude of this third metric. However, many bubble charts are rendered without this correction.
A bubble chart can be considered a "variation of a scatter plot, in which the data points are replaced with bubbles. This type of chart can be used instead of a Scatter chart if your data has three data series, each of which contains a set of values”.
For example, the worksheet in the following picture contains values for three types of data: number of products, dollar value of sales, and percentage size of market share.
Worksheet data to create a Bubble chart
In a Bubble chart, the size of the bubbles is determined by the values in the third data series. For example, the following Bubble chart displays bubble sizes that correspond to the values in the third column of the sample data (Market share %).
Picture of a Bubble chart
Notice that the data in this Bubble chart is plotted as follows:
  • Number of products is displayed along the horizontal (x) axis.
  • Sales amounts are displayed along the vertical (y) axis.
  • Market share percentages are represented by the size of the bubbles.

When to use a Bubble chart?
Bubble charts are often used to present financial data. Bubble charts can facilitate the understanding of the social, economical, medical, and other scientific relationships
We should use a bubble chart Bubble, when we want specific values to be more visually represented in your chart by different bubble sizes. Bubble charts are useful when your worksheet has any of the following types of data:
  • Three values per data point     Three values are required for each bubble. These values can be in rows or columns on the worksheet, but they must be in the following order: x value, y value, and then size value.
  • Negative values     Bubble sizes can represent negative values, although negative bubbles do not display in the chart by default. You can choose to display them by formatting that data series. When they are displayed, bubbles with negative values are colored white (which cannot be modified) and the size is based on their absolute value. Even though the size of negative bubbles is based on a positive value, their data labels will show the true negative value.
  • Multiple data series     Plotting multiple data series in a Bubble chart (multiple bubble series) is similar to plotting multiple data series in a Scatter chart (multiple scatter series). While Scatter charts use a single set of x values and multiple sets of y values, Bubble charts use a single set of x values and multiple sets of both y values and size values.

When we create a bubble chart, we can choose one of the following bubble chart subtypes.
Bubble or bubble with 3-D effect - both bubble chart types compare sets of three values instead of two. The third value determines the size of the bubble marker. You can choose to display bubbles in 2-D format or with a 3-D effect.

Gaurav Kumar
Marketing 2

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