This is a good question. You can use define_metric to define a custom x-axis.
import wandb

wandb.init()
# define our custom x axis metric
wandb.define_metric("custom_step")
# define which metrics will be plotted against it
wandb.define_metric("validation_loss", step_metric="custom_step")

for i in range(10):
  log_dict = {
      "train_loss": 1/(i+1),
      "custom_step": i**2,
      "validation_loss": 1/(i+1)   
  }
  wandb.log(log_dict)


  
      

      docs.wandb.ai
  

  
    

Log Data with wandb.log

  Keep track of metrics, videos, custom plots, and more