When should I use a log scale?

When your values span several orders of magnitude, or when you expect exponential or power-law behaviour. A log scale makes multiplicative change look linear and keeps small values visible next to large ones.

How do I find the doubling time from a semi-log plot?

Fit an exponential trendline. The equation y = a·e^(bx) gives the rate b; the doubling time is ln(2) / b, in the same units as the X axis.

How do I make a semi-log graph?

Tick "Log Y" under Axes. The Y axis switches to powers of ten with minor ticks at 2–9.

Can I set the log axis range?

Yes. Enter a min and max (both above zero). By default the axis extends to whole decades.

No. Your data never leaves your browser. Plotiq parses and plots files locally with JavaScript, so nothing is uploaded to a server.

Log Graph Maker – Semi-Log and Log-Log Plots Online

Plot data on a logarithmic scale. Use a log Y axis for growth and decay, or log on both axes for power laws.

Series	n	Mean	Median	SD	SE	Min	Max
OD₆₀₀	8	0.27275	0.142	0.31827	0.11252	0.02	0.931

Series	Model	Equation	Parameters	R²
OD₆₀₀	exp	y = 0.01998e^(0.5495x)	a = 0.019976, b = 0.54946	0.99997

How to use the Log Graph Maker

Paste your data.
Under Axes tick "Log Y" for a semi-log plot, or both "Log X" and "Log Y" for log-log.
Add an exponential fit (semi-log) or power fit (log-log) if you need one.
Download the graph.

Semi-log plots

On a semi-log graph (log Y, linear X), exponential growth or decay appears as a straight line. That makes it easy to see the exponential phase of bacterial growth, radioactive decay, or first-order reaction kinetics, and to spot where the data leaves that behaviour.

Log-log plots

With both axes logarithmic, a power law y = ax^b becomes a straight line with slope b. Log-log plots are used for scaling laws, frequency responses and data spanning several orders of magnitude.

Zero and negative values

A logarithm of zero or a negative number is undefined, so those points are skipped on a log axis. If your data contains zeros, consider adding a small offset or using a linear axis.

Worked example: bacterial growth

The sample data is optical density (OD₆₀₀) measured every hour during the exponential growth phase. On a linear axis the curve bends upwards; on a log Y axis the same points fall on a straight line. With an exponential trendline, Plotiq reports y = a·e^bx. The growth rate is b per hour, and the doubling time is ln(2) / b. For this data b ≈ 0.55 h⁻¹, so the culture doubles roughly every 1.3 hours.

You want	Axes	Fit	Straight line means
Growth or decay rate	Log Y	Exponential	Constant percentage change per unit X
Scaling law / exponent	Log X and log Y	Power	y = a·x^b; slope = b
Diminishing returns	Log X	Logarithmic	y = a + b·ln x

Reading values off a log axis

Equal distances on a log axis mean equal ratios, not equal differences: the gap from 1 to 10 is the same as from 10 to 100. Minor ticks between two powers of ten mark 2×, 3×… 9× that power, so the tick just above 0.1 is 0.2. Hover over any point to read its exact value instead of estimating it from the axis.

Frequently asked questions

When should I use a log scale?: When your values span several orders of magnitude, or when you expect exponential or power-law behaviour. A log scale makes multiplicative change look linear and keeps small values visible next to large ones.
How do I find the doubling time from a semi-log plot?: Fit an exponential trendline. The equation y = a·e^(bx) gives the rate b; the doubling time is ln(2) / b, in the same units as the X axis.
How do I make a semi-log graph?: Tick "Log Y" under Axes. The Y axis switches to powers of ten with minor ticks at 2–9.
Can I set the log axis range?: Yes. Enter a min and max (both above zero). By default the axis extends to whole decades.
Is my data private?: No. Your data never leaves your browser. Plotiq parses and plots files locally with JavaScript, so nothing is uploaded to a server.

Last updated: 1 November 2026 · by the Plotiq team