I'm currently writing a business report and set up a single column design with many diagrams of varying sizes throughout the document.
I was wondering, which alignment is most appropriate, and why?
In small texts and long columns, there is not much difference in terms of readability. A report is not a novel, sometimes the text is just as a reference.
Stan already commented the importance of the line spacing in this case.
I am noticing something else about your spacings.
As you arranged this, you produced a gap between the graph and the paragraph. I am assuming this text is referencing to the graph, so this gap produced by the left alignment is not good.
In this case, it could be used a right align!
So. You need to consider:
How many words a row have?
How long is the paragraph?
What is the space between rows?
Is the document text only or have a lot of elements?
What font are you using?
An extra tip. Try using a generous margin. The document looks cleeaner.
Simplistically, flush-left text is easy to read as the word spacing is regular at the expense of the line length. Hyphenating your text will help recover the look of regularity at the expense of readability.
Justified text looks regular, formal, and organized by compromising the word spacing for the normalized line length.
You have little choice but running your illustrations, tables, graphs, and figures as they are referenced. See how they look to you flush-left or centred between the margins.
More important than the right-hand margin is the line spacing. Lighten the heavy look of the paragraphs by increasing the space between the lines of text (leading/line-spacing) slightly. From 1.2 to 1.25 x font size, say.
Justification makes sense for wide columns of text, so it could work in your case. This is ultimately for you to decide between left alignment and justification, depending on the overall content volume and how your text integrates with the charts. Personally i will always try to avoid justification and hyphenation, and i have done dozens of similar documents for clients.
And here's a good way to work with the charts.