Simply put, the more data you have, the more reliable the visualization because it is based on more data points, i.e., a larger sample. For example, when I buy a product online I often look at the reviews to see what others who have purchased the product think of it. In my opinion, the more people who review the product the better, because then I know that the overall rating is based on a large number of people and may be more “trustworthy” than a product with only a handful of reviews. Is a 4.5 rating (on a scale of 1 to 5) better if 10 people rated it, or 1,000 people rated it? I would prefer 1,000 ratings.
The same is can be true when displaying data in a visualization. That is not to say that a line chart with only 10 data points is not valid or incorrect, because that is not the case at all. The visualization will still illustrate the trends in the data, but if you had an additional 990 data points on that line chart you would have a better representation of the data and a more valuable visualization.
If you look at the visualization below, the chart on the left has 16 data points and the chart on the right has 1,000 data points. The 16 data points used in the first chart are also present in the 1,000 data point chart, but the stories they tell are different. In the chart on the left, the average sales varies over the years, with the highest average sales being in 2012 at $93 and the lowest being $3 in 2007. Whereas in the 1,000 data point chart, the average sales year to year tends to be between $42 and $54. Both charts are accurate in displaying the data, but the 1,000 data point chart provides more information and a more accurate look at the sales over the years. So when telling the story these visualizations are illustrating, it is important to put them in context and to note the size of the data set that you are working with. With only a few sales figures, the chart on the left is not as rich in information when illustrating the sales trends over 15 years as the chart on the right is.
In this example, more is better. The larger data set provides a richer chart. When it comes to visualizations, the data is everything. Without data, there cannot be a visualization. But it is also important to remember the context when telling the story. When designing, less is more. When it comes to the data, the more the merrier.