The graph below charts a century of church growth from 1910 to 2010 based on data from the Atlas of Global Christianity, with each dot at the intersection between its 1910 and 2010 populations. To make the graph more readable, it's presented in log form.
The line in the middle represents points where the 1910 and 2010 populations are the same--thus, stagnation. This is not common, as most Christian populations have increased if only by demographic growth (birth-death). But it does happen: France, for example, had 4.09 million believers in 1910, and has 4.19 million today: an increase of just 100,000, barely enough to show up on the chart.
Populations "under" the line are those that have lost populations. (This, too, is less common than one might think.) One example is Algeria, which had close to 650,000 Christians in 1910, and today has just over 100,000. Another example is Turkey, which sank from 3.3 million Christians in 1910 to under 200,000 today.
To the far left, you can see a handful of dots going up the graph. These are countries that had no Christians in 1910, and now have significant populations. They show up where they do because they are at 'zero' on the 1910 scale. These include countries like Bhutan, Nepal, and Chad.