Customize log axes

Use define_metric() to set a custom x-axis for logged metrics instead of the default W&B step counter.

Set a custom x-axis when you log metrics to W&B. By default, W&B logs metrics as steps. Each step corresponds to a wandb.Run.log() API call.

For example, the following script has a for loop that iterates 10 times. In each iteration, the script logs a metric called validation_loss and increments the step number by 1.

import wandb

with wandb.init() as run:
  # range function creates a sequence of numbers from 0 to 9
  for i in range(10):
    log_dict = {
        "validation_loss": 1/(i+1)   
    }
    run.log(log_dict)

In the project’s workspace, the validation_loss metric is plotted against the step x-axis, which increments by 1 each time wandb.Run.log() is called. From the previous code, the x-axis shows the step numbers 0, 1, 2, …, 9.

In certain situations, it makes more sense to log metrics against a different x-axis such as a logarithmic x-axis. Use the define_metric() method to use any metric you log as a custom x-axis.

Specify the metric that you want to appear as the y-axis with the name parameter. The step_metric parameter specifies the metric you want to use as the x-axis. When you log a custom metric, specify a value for both the x-axis and the y-axis as key-value pairs in a dictionary.

Copy and paste the following code snippet to set a custom x-axis metric. Replace the values within <> with your own values:

import wandb

custom_step = "<custom_step>"  # Name of custom x-axis
metric_name = "<metric>"  # Name of y-axis metric

with wandb.init() as run:
    # Specify the step metric (x-axis) and the metric to log against it (y-axis)
    run.define_metric(step_metric = custom_step, name = metric_name)

    for i in range(10):
        log_dict = {
            custom_step : int,  # Value of x-axis
            metric_name : int,  # Value of y-axis
        }
        run.log(log_dict)

As an example, the following code snippet creates a custom x-axis called x_axis_squared. The value of the custom x-axis is the square of the for loop index i (i**2). The y-axis consists of mock values for validation loss ("validation_loss") using Python’s built-in random module:

import wandb
import random

with wandb.init() as run:
    run.define_metric(step_metric = "x_axis_squared", name = "validation_loss")

    for i in range(10):
        log_dict = {
            "x_axis_squared": i**2,
            "validation_loss": random.random(),
        }
        run.log(log_dict)

The following image shows the resulting plot in the W&B App UI. The validation_loss metric is plotted against the custom x-axis x_axis_squared, which is the square of the for loop index i. Note that the x-axis values are 0, 1, 4, 9, 16, 25, 36, 49, 64, 81, which correspond to the squares of 0, 1, 2, ..., 9 respectively.

You can set a custom x-axis for multiple metrics using globs with string prefixes. As an example, the following code snippet plots logged metrics with the prefix train/* to the x-axis train/step:

import wandb

with wandb.init() as run:

    # set all other train/ metrics to use this step
    run.define_metric("train/*", step_metric="train/step")

    for i in range(10):
        log_dict = {
            "train/step": 2**i,  # exponential growth w/ internal W&B step
            "train/loss": 1 / (i + 1),  # x-axis is train/step
            "train/accuracy": 1 - (1 / (1 + i)),  # x-axis is train/step
            "val/loss": 1 / (1 + i),  # x-axis is internal wandb step
        }
        run.log(log_dict)