Wind graph changing sizes with time steps, caused by min/max/mean watermark

### Describe the bug

Best shown in video here:
https://drive.proton.me/urls/V4547AZ13C#ZuURhKur1lmE

Basically, there are min/max/mean values that are formatted with `.3g` which is variable depending on number of nonzero digits. The watermark, when having more digits in min/max/mean, "pushes" back the main image, making it smaller.

The watermark should be formatted with `.3f` instead to fix the decimal digits for all timesteps. Or make the watermark floating.

### How to reproduce

A simple vector plot with time steps:
```yaml
- operator: plot.vector_plot
series_coordinate: time
cube_u:
operator: filters.filter_cubes
constraint:
operator: constraints.generate_var_constraint
varname: x_wind
cube_v:
operator: filters.filter_cubes
constraint:
operator: constraints.generate_var_constraint
varname: y_wind
```

### Expected behaviour

No jumping.

Start at the plot.vector_plot entry point and reproduce the issue with the YAML vector plot and time steps provided. Inspect how the min/max/mean watermark is formatted and positioned; done means the wind graph no longer changes size or jumps between timesteps.

Written by the indexing model from the issue text.