# Gadfly plot failing (suspect numerical issues)
**URL:** https://discourse.julialang.org/t/gadfly-plot-failing-suspect-numerical-issues/53576
**Category:** Visualization
**Tags:** gadfly
**Created:** [January 19, 2021, 2:29am UTC](https://discourse.julialang.org/t/gadfly-plot-failing-suspect-numerical-issues/53576 "2021-01-19T02:29:12Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@JonT](https://discourse.julialang.org/u/JonT)
#### Post date: [January 19, 2021, 2:29am UTC](https://discourse.julialang.org/t/gadfly-plot-failing-suspect-numerical-issues/53576/1 "2021-01-19T02:29:12Z")
I have a situation where I am trying to plot errors, which are small but are nowhere near subnormal numbers in Float64.
Unfortunately, `plot()` is producing an incorrect graph, using Gadfly 1.3.1 in Julia 1.5.3.
Here is a simple demonstration of problem:
```julia
using Gadfly;
xvals = range( 0.0, 1.0, length=20 );
yvals = [0.0 , 1.5770373161480791e-18 , 2.983115103490406e-18 , 4.197248008233336e-18 , 5.3945317696645016e-18 , 6.844316881913259e-18 , 8.708236700751923e-18 , 1.0863668278404468e-17 , 1.2894457644236792e-17 , 1.4317750840733128e-17 , 1.4965586158467704e-17 , 1.5276181431484313e-17 , 1.6182869477102057e-17 , 1.8443012277801592e-17 , 2.1692238089739158e-17 , 2.4126051771260834e-17 , 2.4010574742241276e-17 , 2.306119442978225e-17 , 2.6917955234178657e-17 , 2.575849671298423e-17];
# this plots a line of zeros along the x axis
plot( x = xvals, y = yvals )
# this (still) plots a line of zeros
plot( x = xvals, y = yvals * 8 )
# this plots something that looks right, but its choice of yscale labels is strange
# and of course the scale is not correct :(
plot( x = xvals, y = yvals * 9 )
# this plots the correct shape
# but of course the scale is not correct :(
plot( x = xvals, y = yvals * 10 )
```
While I have worked around the problem for this data set, it raises the issue that the graphs are not necessarily reliable, which is obviously troubling for future use.
---
### Author:  [@oheil](https://discourse.julialang.org/u/oheil)
#### Post date: [January 19, 2021, 9:16am UTC](https://discourse.julialang.org/t/gadfly-plot-failing-suspect-numerical-issues/53576/2 "2021-01-19T09:16:24Z")
> [@JonT](#):
>
> it raises the issue that the graphs are not necessarily reliable, which is obviously troubling for future use
A good graph never comes out of the box automatically out of any box.
It’s not the tool I have to rely on, whenever I have to interprete a graph, but it is the creator of the graph, which is you, in this case.
Having said this and out of the way, I would recommend to change the order of magnitude of the y axis by 10^17, e.g. with:
```julia
plot( x = xvals, y = yvals * 10^17 , Guide.ylabel("y x 10^17"))
```
This is much easier to comprehend by the box (Gadfly) and by the viewer.
Regarding this
```julia
# this plots a line of zeros along the x axis
plot( x = xvals, y = yvals )
```
there obviously takes place some rounding or something similar for the purpose of pretty printing the numbers on the y axis. If you want more reliability from Gadfly in the future, you should open an issue about this at [Issues · GiovineItalia/Gadfly.jl · GitHub](https://github.com/GiovineItalia/Gadfly.jl/issues) . A short search didn’t come up with an existing issue.
---
### Author:  [@Mattriks](https://discourse.julialang.org/u/Mattriks)
#### Post date: [January 19, 2021, 11:39am UTC](https://discourse.julialang.org/t/gadfly-plot-failing-suspect-numerical-issues/53576/3 "2021-01-19T11:39:49Z")
Gadfly uses the function [`optimize_ticks`](https://github.com/GiovineItalia/Gadfly.jl/blob/21d8674eaa48d54037ff5ed998141569d39efee0/src/ticks.jl#L36-L52) to work out the axis ticks.
```julia
import Gadfly: optimize_ticks
# yvals from above example
sc = 9
# sc = 1e17
extrema(yvals*sc)
optimize_ticks(extrema(yvals*sc)...)
```
Improvements for small numbers welcome!
---
### Author:  [@oheil](https://discourse.julialang.org/u/oheil)
#### Post date: [January 19, 2021, 1:27pm UTC](https://discourse.julialang.org/t/gadfly-plot-failing-suspect-numerical-issues/53576/4 "2021-01-19T13:27:29Z")
I didn’t check in detail and didn’t tested it, but it seems to me that the usage of
```julia
eps()
```
in function `optimize_ticks_typed` in `ticks.jl` line 66 may be the problem.
For `Float64` `eps()` isn’t a constant, but dependend of the value itself.
So for `Numbers` I would propose to use a variable `e` which is calculated as:
```julia
e = maximum((eps(x_min),eps(x_max)))
```
which is in the case of above `yvals`:
```julia
julia> e = maximum((eps(minimum(yvals)),eps(maximum(yvals))))
3.0814879110195774e-33
```
Compare this to:
```julia
julia> eps()
2.220446049250313e-16
```
What do you think? Does this make sense?
---
### Author:  [@JonT](https://discourse.julialang.org/u/JonT)
#### Post date: [January 20, 2021, 4:49am UTC](https://discourse.julialang.org/t/gadfly-plot-failing-suspect-numerical-issues/53576/5 "2021-01-20T04:49:24Z")
Thank you!
Looking at the code, I conjecture it needs to be used in two places in `optimize_ticks_typed()` in `ticks.jl`
The following had my graphs working.
line 73
```julia
one_t = oneunit(T)
- if x_max - x_min < eps() * one_t
+ epsilon = eps( max( abs(x_min), abs(x_max) ) )
+ if x_max - x_min < epsilon * one_t
R = typeof(1.0 * one_t)
return R[x_min], x_min - one_t, x_min + one_t
end
```
line 100
```julia
stp = q*10.0^z
- if stp < eps()
+ if stp < epsilon
continue
```
But a sample of one is not testing.