There's a quiet moment of clarity in this question, and it deserves more than a quick fix. When a linear trend line cooperates but an exponential one vanishes into the chart's blank space, the problem isn't the data or the user's skill. It's the math refusing to play along with the visual story we've asked it to tell. Exponential projections grow fast, sometimes so fast that the scale of the chart stretches, and the line simply leaves the visible plot area. That's not a failure in your approach; it's a signal that the tool is showing you something true about the underlying trend, even if it's inconvenient for a clean picture.
For the person who posted this, apologetic, new to Excel, eager to learn, the practical takeaway is simple: your chart isn't broken, and neither are you. The linear line works because it's gentle, predictable, and fits comfortably within the axes. The exponential line disappears because it's not supposed to fit comfortably. It's meant to climb, and climb hard. When that happens, the fix isn't to abandon the exponential model. It's to adjust the scale, consider a logarithmic axis, or even separate the two trend lines into their own charts so each can breathe. The data isn't wrong; the presentation just needs to catch up to the reality of the numbers.
This moment matters beyond one user's scatterplot. It's a reminder that spreadsheets are not neutral tools. They reflect the choices we make about what to look at and how. A linear trend line says "steady progress." An exponential one says "watch out, because this is accelerating." When one of them vanishes, we're forced to confront which story we actually want to tell. And for anyone working with data, new or experienced, that's a valuable question to sit with. Don't let a disappearing line push you toward the easier option. Let it push you toward understanding why it disappeared.
So here's the concrete next step: check the axis bounds, try a secondary axis for the exponential series, or plot the projections on a separate chart with a log scale. You'll see the line reappear, and you'll also see the future it's drawing, whether that's a steady climb or a steep curve. That's not a technicality. That's the whole point of making projections visible in the first place.