📚 Constructing Bar Charts and Histograms in Biology | 生物学科中条形图与直方图的构建
Accurate graphical presentation is a core practical skill in Cambridge A-Level Biology. This article explains how to construct bar charts and histograms correctly, when to use each type, and how to avoid common errors.
准确的图表呈现是 Cambridge A-Level 生物课程的核心实验技能。本文将解释如何正确构建条形图和直方图、何时使用每种图表，以及如何避免常见错误。
1. The Role of Graphs in Biological Data | 图表在生物数据中的作用
Biologists collect numerical data from experiments, surveys and field work. A well-drawn graph reveals patterns, trends and anomalies more clearly than a table alone.
生物学家从实验、调查和野外工作中收集数值数据。绘制良好的图表比单纯的数据表更能清晰地揭示模式、趋势和异常值。
In Cambridge assessments, marks are awarded for choosing the correct graph, labelling axes with units, using an appropriate scale, and plotting data accurately.
在 Cambridge 考试中，选择正确的图表、用单位标注坐标轴、使用合适的刻度并准确绘制数据都会得到分数。
Graphs also allow scientists to compare groups, describe variation, and evaluate whether experimental results support a hypothesis.
图表还能让科学家比较不同组别、描述变异，并评估实验结果是否支持假设。
2. Discrete and Continuous Variables | 离散变量与连续变量
The first decision is to identify the independent variable. Discrete variables have separate categories or countable whole numbers, such as blood group, antibiotic type or number of leaves.
首先要确定自变量。离散变量具有独立的类别或可数的整数，例如血型、抗生素种类或叶片数量。
Continuous variables can take any value within a range, such as leaf length, body mass, temperature or enzyme activity.
连续变量可以在一个范围内取任意值，例如叶片长度、体重、温度或酶活性。
Discrete and categorical data are usually shown in a bar chart; continuous data are usually shown in a histogram or line graph.
离散和分类数据通常用条形图表示；连续数据通常用直方图或线图表示。
Choosing the wrong graph type loses information and often loses marks, so this classification should always be done before plotting.
选择错误的图表类型会丢失信息，并且通常会丢分，因此在绘图前应始终先进行分类。
3. Bar Charts: Definition and Construction Rules | 条形图：定义与构建规则
A bar chart displays frequencies, means or other summary values for separate categories. The bars do not touch because the categories are distinct.
条形图显示不同类别的频数、平均值或其他汇总值。由于类别是独立的，条形之间不相接触。
Construction rules: label both axes, put the categorical variable on the x-axis, start the y-axis at zero, keep bar widths equal, and leave equal gaps between bars.
构建规则：标注两条坐标轴；将分类变量放在 x 轴；y 轴从零开始；条形宽度相等；条形之间留出相等的间隙。
For example, the mean inhibition zone diameter for three antibiotics can be plotted as three separate bars with clear labels.
例如，三种抗生素的平均抑菌圈直径可以绘制为三个独立的条形，并带有清晰标签。
Do not use a bar chart for a continuous variable, because this hides the order and shape of the distribution.
不要对连续变量使用条形图，因为这会隐藏分布的先后顺序和形状。
Bar charts may show counts, percentages, means or rates, but whichever quantity is used must be stated clearly on the y-axis.
条形图可以显示计数、百分比、平均值或速率，但无论使用哪种量，都必须在 y 轴上清楚标明。
4. Histograms: Definition and Construction Rules | 直方图：定义与构建规则
A histogram represents the distribution of a continuous variable. The data are grouped into class intervals, and bars touch to show that the variable is continuous.
直方图表示连续变量的分布。数据被分组到组距中，条形相互接触以表示变量是连续的。
Construction rules: choose intervals of equal width where possible, plot the class intervals on the x-axis, plot frequency or frequency density on the y-axis, and ensure bars touch.
构建规则：尽可能选择等宽的组距；在 x 轴上标出组距；在 y 轴上标出频数或频率密度；确保条形相互接触。
For example, the lengths of 100 ivy leaves can be grouped into 10 mm intervals to show the distribution of leaf size.
例如，100 片常春藤叶的长度可以按 10 mm 的组距分组，以显示叶片大小的分布。
When intervals are equal, the height of each bar represents frequency. When intervals are unequal, frequency density must be plotted instead.
当组距相等时，每个条形的高度代表频数。当组距不相等时，必须绘制频率密度。
5. Choosing Class Intervals for Histograms | 直方图组距的选择
The number of class intervals affects the shape of the histogram. Too few intervals hide important detail; too many create a noisy, uneven pattern.
组距的数量会影响直方图的形状。组距太少会隐藏重要细节；组距太多会产生杂乱且不均匀的图形。
A common guideline is to use 5 to 15 intervals. The interval width should be easy to read, such as 5, 10 or 20 units, depending on the range.
一个常用原则是使用 5 到 15 个组距。组距宽度应易于读取，例如根据全距选择 5、10 或 20 个单位。
All intervals must be continuous and non-overlapping. There must be no gaps between intervals such as 10-19 and 20-29.
所有组距必须连续且不重叠。组距之间不能有间隙，例如 10-19 和 20-29。
Class boundaries should be written clearly. For example, use intervals like 10-19, 20-29 and 30-39 rather than overlapping or ambiguous labels.
组距边界应书写清楚。例如，使用 10-19、20-29 和 30-39 这样的组距，而不是重叠或含糊的标签。
6. Frequency Density for Unequal Intervals | 不等组距下的频率密度
If class intervals have different widths, plotting frequency directly is misleading because wider intervals naturally contain more values.
如果组距宽度不同，直接绘制频数会产生误导，因为更宽的组距自然会包含更多数值。
In such cases, plot frequency density using the equation:
在这种情况下，应使用以下公式绘制频率密度：
frequency density = frequency ÷ class width
频率密度 = 频数 ÷ 组距宽度
The area of each bar is then proportional to frequency, which makes the histogram valid even with unequal intervals.
这样每个条形的面积与频数成正比，即使组距不等，直方图也仍然有效。
Always calculate frequency density before plotting if the class widths are not all the same.
如果各组距宽度不完全相同，绘图前一定要先计算频率密度。
7. Axes, Scales and Label Conventions | 坐标轴、刻度与标注规范
Every graph must have a title that states what is plotted. Axes should be labelled with the variable name and the unit, for example 'Leaf length / mm'.
每张图都必须有标题，说明绘制的内容。坐标轴应标注变量名称和单位，例如 '叶片长度 / mm'。
Use a linear scale that spreads the data over at least half of the graph paper or plot area. Avoid awkward multiples such as 3, 7 or 11 per division.
使用线性刻度，使数据至少占据方格纸或绘图区域的一半。避免每个分格使用 3、7 或 11 等不便的倍数。
The y-axis should usually start at zero. If it does not, a scale break must be shown clearly, though this is rarely needed in A-Level biology.
y 轴通常应从零开始。如果没有从零开始，则必须清楚显示刻度中断，但在 A-Level 生物中很少需要这样做。
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