This page discusses how to make a successful bar chart, including:
Bar charts are excellent for comparing quantitative data across different categories.
Strive for clarity: The title of your chart should be descriptive and tell the viewer what to look for. You should label your axes, including identifying appropriate units of measurement. If you have only one series of bars, you should probably make them all the same color, unless you want to draw specific attention to one or two of them.
Include what's necessary: Make sure you don't add too much extraneous "ink" on the page. Instead of using dark gridlines, can you lighten them or erase them altogether? Do you need to label the amount directly on each bar, or is an estimated comparison enough? If you label each bar's amount, can you remove most of the numbers from the corresponding axis?
Orient the bars: Note that the bars can be displayed either vertically or horizontally. Which direction makes it easier to compare your data? If the category labels are long, you may want to make horizontal bars so the labels can also remain horizontal.
Sort the bars: You also want to consider how to sort your bars. Does it make sense to show them biggest to smallest? Do your categories naturally flow in an order you want to preserve for your viewer? For instance, will your viewer expect to see them alphabetically?
Lying through omission: Don't ever chop off the bottoms of your bars. Bar charts must always, always start at 0 on the numerical axis. The purpose of visualization is to make insight clearer, so don't make it harder to see the underlying data. If you create a bar chart and the bars show almost no difference in height, there are other approaches to take than to chop off the bottom. Perhaps it makes sense to normalize the amounts to one category, and show each other category's diverging amounts, both positive and negative, from that first category. Perhaps you need a different kind of chart to represent your data. Or perhaps what you've found is that there simply isn't much happening in your data, and that's okay too!
Unnecessary 3D: It's also important to avoid using 3D. This is true of all of the charts discussed in this guide. Adding a third dimension to a two-dimensional chart actually makes it harder for a viewer to understand, because it distorts the image. You should only use 3D when the third dimension explicitly adds new information (like showing a 3D model of a building). If you truly have another dimension you want to show, which will need a 3D bar chart, you still should avoid it. The information will be clearer to your viewer if you use two separate bar charts, or consider a different kind of chart that is meant to show multiple different variables.
Below is an example of a really bad bar chart.
Possibly number one is the problem that there is no chart title, and in fact no text of any kind. (What is the chart about?) In addition to these problems, notice the unnecessary use of 3D, which distorts the size of the bars. There is also unnecessary color, including the use of color gradients. What information do these components add? They add nothing, and in fact they distract from what the data could show through a good bar chart.
Color: One important consideration for accessibility is the use of color. There are several different types of color-blindness, which can affect how well your audience reads your chart. The most common is red-green color-blindness, which means that red and green look very similar. As much as you may want to use green to mean something positive and red to mean something negative, you should pick a different pair of colors instead. The websites ColorBrewer, Contrast-A, and Viz Palette are great tools for identifying colors that work well together and also work for color-blind audiences. You can also check your finished product against a color blindness simulator. You may also want to test how your chart looks in black and white, either by printing it out or using your chart creation tool to transform the colors to grayscale.
Text: It's also important to ensure that any text on your chart is easy to read, which is affected by both the size of the text and the choice of font. Try to avoid "pretty" fonts; it's best to use something sans serif like Ariel or Calibri. When designing your chart, try to keep all text horizontal (nobody wants to have to tilt their head to read). Make sure your chart title and any labels are descriptive and clear.
Embedding an image: If you are embedding the chart as an image in a document or online, you should also include an "alt" tag that describes what the chart shows. If you are able to share the data behind the chart, it is also recommended to provide a descriptive link to the data near the chart image.