A decluttered chart is quiet. That's what you built it for, and it's also its weak spot, because a quiet chart lets a reader arrive at a reading you didn't intend. The fix is an annotation. It's a short sentence placed on the plot, next to the mark it explains, in the accent's colour or in body ink.
Worked example: on-time delivery at Meghna Logistics
Meghna Logistics' on-time delivery rate over twelve months makes the case. The series runs 88.4, 87.9, 86.2, 82.1, 79.6, 81.3, 84.7, 88.9, 90.2, 91.1, 91.6 and 92.4 percent, from April to March. Full-year change is +4.0 points. A summary table would carry that number, and it's the least interesting thing on the chart. The trough is 79.6% in August. From there to March the recovery is 12.8 points over seven months, an average of 1.83 points a month.
Give a reader the line and nothing else and they'll see a dip and a recovery. They'll have no idea whether anything caused either. Two annotations fix that, and neither is longer than a line:
Aug: monsoon flooding closed the Bhiwandi hub for nine days. Recovery has averaged 1.83 points a month since; four of twelve months now clear the 90% target.
The first turns an anomaly into a known event, so a reader stops treating the whole series as noisy. The second states the rate of the recovery, which a reader would otherwise have to work out from the chart. It also names the target, so "90%" lands in the reader's head as a threshold. Without it, it's just a number on an axis.
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What are the three rules for a useful annotation?
Three rules keep annotations useful.
An annotation says what to see. It doesn't describe what's there. "Sales fell in Q3" repeats the chart. "The Q3 fall is entirely the Bhiwandi closure; every other hub was flat" adds what the chart can't show. It's the sentence a reader would otherwise ask you for.
An annotation goes next to its mark. A caption below the plot forces the same round trip a legend does. Where the annotation refers to a point, put it within about one label's width of that point and let a thin leader line close the gap.
An annotation is held to the same honesty rule as a title. If the recovery is 1.83 points a month, don't write "rapid". If four months of twelve clear the target, don't write "consistently above target". One overstated annotation costs more credibility than a cluttered chart ever will. A reader who catches one stops believing the rest.
When is a reference line better than an annotation?
The reference line is annotation's quiet cousin, and it often does the same job for less. A horizontal rule at the 90% target, labelled once at the right-hand end, replaces eleven gridlines and tells the reader what "good" is. A vertical rule at the month a policy changed does the same for time. Use them wherever the reader needs a comparison against a constant.