# How to plot a spectrum with dB as y-axis with PythonPlot
**URL:** https://discourse.julialang.org/t/how-to-plot-a-spectrum-with-db-as-y-axis-with-pythonplot/100172
**Category:** Visualization
**Tags:** question, package, plotting, pythonplot
**Created:** [June 11, 2023, 7:44am UTC](https://discourse.julialang.org/t/how-to-plot-a-spectrum-with-db-as-y-axis-with-pythonplot/100172 "2023-06-11T07:44:38Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@ufechner7](https://discourse.julialang.org/u/ufechner7)
#### Post date: [June 11, 2023, 7:44am UTC](https://discourse.julialang.org/t/how-to-plot-a-spectrum-with-db-as-y-axis-with-pythonplot/100172/1 "2023-06-11T07:44:38Z")
I am using PythonPlot and want to have a logarithmic y axis formatted in dezibel (dB). In Python I could use the following code:
```python
from UliEngineering.Math.Decibel import *
import matplotlib.ticker as mtick
def decibel_formatter(v0=1.0, unit='dB'):
def format_value(value, pos=None):
dB = value_to_dB(value, v0=v0)
return f'{dB:.0f} {unit}'
return format_value
# Usage example:
plt.gca().set_yscale("log") # Optional
plt.gca().yaxis.set_major_formatter(mtick.FuncFormatter(decibel_formatter()))
```
Source: [https://techoverflow.net/2023/03/13/how-to-format-axis-as-db-decibel-using-matplotlib/](https://techoverflow.net/2023/03/13/how-to-format-axis-as-db-decibel-using-matplotlib/)
How can I do this in Julia? In particular, how can I write a custom formatter in Julia?
---
### Author:  [@ufechner7](https://discourse.julialang.org/u/ufechner7)
#### Post date: [June 11, 2023, 8:01am UTC](https://discourse.julialang.org/t/how-to-plot-a-spectrum-with-db-as-y-axis-with-pythonplot/100172/2 "2023-06-11T08:01:15Z")
What I have so far:
```julia
using ControlSystemsBase, PythonPlot
function todb(mag)
20 * log10(mag)
end
P = tf(0.01934, [1, 0.01934])
w = exp10.(LinRange(-6, 1, 200));
mag, w1 = bode(P, w)
mag = mag[:]
db = true
if db
plot(w, todb.(mag))
ax = gca()
ax.set_xscale("log")
ylabel("Magnitude [dB]", fontsize = 14)
else
loglog(w, mag)
end
xlabel("Frequency [rad/s]", fontsize = 14)
grid(true, which="both", ls="-.")
nothing
```
That’s good enough even without custom formatter…