Graphing

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Graphing

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Types of Graphs

• Pie Charts
• XY Graphs
• Bar Charts

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What type of graph to use?

• Pie Charts
• Limited in their applicability
• Can only be used when you have a quantitative
  variable associated with a list of categories
  where BOTH the categories and the quantities each
  add up to a WHOLE
• The categories must be disjoint
• Most common error using a pie chart on a set of
  categories that do not make a whole to use a
  pie chart when the categories overlap
• Do not include the total in your pie chart
• Add percentage data labels

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What type of graph to use?

• XY Graphs
• Should be used when you have a lot of data
  points and the categories along the x-axis are
  numerical
• Use the XY Scatter type and then choose a graph
  option with connected data points, usually
• What Excel calls a line graph is very confusing
  and should essentially never be used (Excel line
  graph always treats the x-axis categorically)

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What type of graph to use?

• Bar Charts
• Very flexible
• Can be used whenever there is a quantitative
  variable associated with the categorical variable
• Great when you have a limited amount of data or
  when you want to compare more than one series
• If you do use a bar chart and have years along
  the x-axis, be sure the years are consecutive

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How to make a pie chart

• ../../Excel_Files/EnergyConsumption2005.xls
• Select both the categories and the values
• The graph needs a descriptive title
• The percentages might need to be increased to
  show one or two decimals

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How to make a Bar Chart

• http//qrc.depaul.edu/Excel_Files/HomeHeating.xls
• Advantages and disadvantages with bar charts
• Main advantage is their succinctness and the
  ability they afford to make comparisons within
  categories and across categories
• Best used in printed works so that a view can
  study them carefully
• Disadvantage is that they sometimes present far
  too much information to view as part of a
  presentation
• It is hard to make a single, clear point with
  them, and presenters tend not to leave them up
  long enough to absorb the information fully
• People often make bar charts with quantitative
  data on the x-axis, but they are not careful
  about the fact that bar charts treats the x-axis
  categorically

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How to Make an XY Graph

• http//qrc.depaul.edu/Excel_Files/LakeMichiganLeve
  ls1988-2000.xls
• Vocabulary increasing/decreasing, absolute
  minimum/maximum, relative minimum/maximum,
  increasing at an increasing rate, decreasing at a
  decreasing rate, periodic
• Be able to write paragraphs which describe what
  the graph is saying, generally going from left to
  right, telling the story of the graph

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Guidelines for making an effective Graph

• What is the purpose of making a graph from this
  data?
• What type of graph should you make?
• Pie
• Bar
• XY scatter (line)
• Decide on a title and consider the Ws (who, what,
  where, and when)
• Legend yes or no?
• Descriptive x-axis label (if applicable)
• Descriptive y-axis label (if applicable)
• Scale (if applicable)
• Source

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How to Describe Graphs

• The graph should be able to stand alone without
  any words to tell the reader what they are
  looking at
• If the reader doesnt understand the graph
  without a caption (or story) then the graph isnt
  very good
• In a paragraph describing the graph, you should
  point out what you want the reader to know about
  the graph

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Critique faulty or misleading graphs
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Critique faulty or misleading graphs
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Critique faulty or misleading graphs
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Y-Axis Scale Effects

• One of the most common ways graphs can be
  misleading
• It has become more and more accepted in the media
  to adjust the y-axis