Choose the right chart depending on whether you want to see differences in size between items, change over time, or the relationship between two items.
Read the shape of a data distribution and how widely it spreads from a histogram and a box plot.
Explain what a heatmap, which shows values by color, and a radar chart, which shows the balance across many items, are used for.
A bar chart (棒グラフ) turns amounts into bar lengths so that items can be compared with each other.
For sales by store or case counts by year, you draw one bar per item or point in time, with its length matching the value. Differences in value show up as differences in length, so you can see at a glance which is larger or smaller. Bars sorted from the largest count down and overlaid with a line showing the cumulative share form a Pareto chart.
AnalogyIt is like lining children up side by side to compare heights: you can tell straight away who is tallest.
Exam points
ExamThe line in a Pareto chart (パレート図) adds up the shares one bar at a time from the leftmost bar and reaches 100% at the right end. It is used to narrow the targets for action down to the few items with the largest counts.
ExamThe growth rate (伸び率) is the increase divided by the base amount. A bar that grew by a greater length does not necessarily have the higher growth rate.
A line chart (折れ線グラフ) plots the value at each point in time and joins the points with a line to show change over time.
The horizontal axis typically holds months or years, and the values at each point are joined in order. A line rising to the right means an increase, one falling to the right means a decrease, and the steeper the slope, the bigger the change. A combination chart puts a bar chart and a line chart on one graph, for instance showing amounts as bars and the movement of a ratio as a line.
AnalogyIf you record your temperature every morning and join the dots, the rise and fall of a fever shows up as the shape of the line.
How to rememberWhere the line bends = the point in time when the direction or pace of change shifted.
Exam points
ExamEven while an amount keeps rising, its ratio to net sales (売上高) can fall. Check the trend of a ratio by doing the division separately, rather than reading it off the amounts.
Mix-upTo compare amounts across items or points in time, use a bar chart (棒グラフ); to follow the direction and pace of change in a value, use a line chart (折れ線グラフ).
A scatter plot (散布図) puts the values of two items on the vertical and horizontal axes and plots each record as a dot to show the correlation between the two items.
How the dots in a scatter plot line up
Rising to the right: positive correlation
Falling to the right: negative correlation
No visible slope: little or no correlation
A tendency for one value to be larger as the other grows is a positive correlation, a tendency for it to be smaller is a negative correlation, and the closer the dots cluster around a straight line, the stronger the correlation. A correlation seen in a scatter plot does not mean that one item causes the other. Replacing the dots with circles whose size shows a third quantity gives a bubble chart.
AnalogyPlot each classmate's study time and test score as one dot, and the direction of the band of dots reveals how the two are related.
How to rememberTo scatter is to sow seeds. How the sown dots spread out tells you how strongly the two items are related.
Exam points
ExamThe chart that shows what kind of relationship exists between two sets of data is the scatter plot (散布図).
ExamA scatter plot whose dots run down from the upper left to the lower right shows a negative correlation (負の相関).
ExamTo see the relationship among three quantities on one graph, a bubble chart (バブルチャート) fits. Two of them go on the vertical and horizontal axes, and the remaining one sets the size of each circle.
A histogram (ヒストグラム) splits numeric data into ranges (classes, 階級) and shows how many data fall into each class (the frequency, 度数) as bar height to reveal the shape of the distribution.
Collect numeric data
Divide into classes
Count per class
Draw the frequencies as bars with no gaps
Make the frequencies the bar heights and line the bars up in class order with no gaps, and the peak of the distribution appears.
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Each bar stands for a range within an unbroken run of numbers, so the bars sit side by side in class order with no gaps. Where the peak sits, which way the shape leans, and whether there is one peak or two tell you which values the data gather around and how widely they spread.
AnalogySort the whole class's heights into 5 cm ranges and stack one block per person in each range, and a mountain of heights takes shape.
How to rememberBars standing shoulder to shoulder = number ranges that run on without a break.
Exam points
ExamThe chart that counts measured values interval by interval and shows each frequency (度数) as bar height is the histogram (ヒストグラム).
Mix-upThe bars of a histogram follow the order of the numeric classes (階級), while the bars of a Pareto chart (パレート図) run from the largest count down.
Mix-upEach bar in a bar chart (棒グラフ) shows the value of one item or point in time, while each bar in a histogram shows how many data fall within a range of continuous numbers.
A box plot (箱ひげ図) uses the values that split data sorted from smallest to largest into four parts of equal count (the quartiles, 四分位数), together with the minimum and maximum, to summarize how the data spread as a box with whiskers.
Value used
Position in the diagram
Minimum
End of the left whisker
First quartile
Left edge of the box
Median
Line inside the box
Third quartile
Right edge of the box
Maximum
End of the right whisker
Counting from the small end, the quartiles are the first quartile, the median (second quartile), and the third quartile. The box spans from the first to the third quartile and holds roughly the middle half of the data, and whiskers extend from both ends of the box to the minimum and the maximum.
AnalogyDescribing a marathon's finish times by the leader, the main pack in the middle and the last runner conveys in a few words how spread out the whole field was.
How to rememberA cat's face seen from the front: the box is the face, and the lines reaching left and right are its whiskers.
Exam points
Mix-upA histogram (ヒストグラム) shows the full shape of the distribution's peak, while a box plot (箱ひげ図) narrows it down to just five values. When several groups are lined up for comparison, box plots take up little space.
Histogram vs. Box plot
Aspect
Histogram
Box plot
Values used
Frequency of each class
Five values: the quartiles plus the minimum and maximum
What you can see
Shape of the peak (number of peaks, skew)
Position of the median and width of the spread
Key differenceBest use
Examining one data set's distribution in detail
Comparing the spread of several groups side by side
A heatmap (ヒートマップ) shows the size of numbers arranged in a table or similar grid as shades of color or hues.
With days of the week as rows and time slots as columns, for example, cells with more visitors are painted darker, so the busy time slots stand out as patches of color. Without reading each number, you can grasp at once where the high and low values lie.
AnalogyOn a thermal camera screen, the warmer parts of a body glow redder, so you can spot the hot areas without reading any numbers.
How to rememberHeat: the hotter (larger) a value, the darker or warmer the color it gets.
A radar chart (レーダーチャート) assigns one evaluation item to each axis radiating from the center and shows the balance among the items through the shape of the polygon joining the values on each axis.
Values grow toward the outer end of each axis, so the wider the polygon spreads, the higher the overall rating, and an axis that dents inward marks a weak item. Overlaying the polygons of several subjects or of the average lets you compare which items differ.
AnalogyScore a restaurant on taste, portion, price, service and cleanliness and draw it as a pentagon, and its strong and weak points show in how the shape leans.
How to rememberThe name comes from its look: lines radiating from the center, like a radar screen.
Exam points
ExamThe chart that lays out ratings for several items to show their balance and their gaps from the average on one graph is the radar chart (レーダーチャート).
If the dots in a scatter plot rise to the right, the correlation is positive, and if they fall to the right, it is negative; a correlation alone does not mean cause and effect.
A histogram shows the shape of a distribution through the frequency of each class, and the position and skew of its peak show how the data gather.
The box of a box plot holds roughly the middle half of the data, and the lengths of the box and whiskers show how large the spread is.
A heatmap shows the size of values by color, and a radar chart shows the balance across many items by the shape of a polygon.