Hi!

I'm trying to make a simple multiplication operation on a flattened field, but I don't seem to be able to access the flattened fields in the calculation.

For instance, if I make this multiplication operation with non-flattened fields, it works fine:

```
format: {
property: aggregations.freqs.freq.buckets
}
}
transform: [
{
calculate: datum.rangos.buckets
as: snr_rangos
}
{
flatten: [
snr_rangos
]
}
{
calculate: datum.key
as: frequencies
}
{
calculate: datum.doc_count
as: devices
}
{
calculate: datum.doc_count * 100
as: result
}
]
```

But if I try to make the same simple operation on the flattened snr_rangos, I get an error: Unrecognized signal name: "snr_rangos":

```
format: {
property: aggregations.freqs.freq.buckets
}
}
transform: [
{
calculate: datum.rangos.buckets
as: snr_rangos
}
{
flatten: [
snr_rangos
]
}
{
calculate: datum.key
as: frequencies
}
{
calculate: datum.doc_count
as: devices
}
{
calculate: snr_rangos.doc_count * 100
as: result
}
]
```

The flatten is working fine, I am perfectly able to use the fields from snr_rangos on the enconding later. This works fine:

```
encoding: {
x: {
field: frequencies
axis: {
title: DVB-C frequency
}
}
y: {
field: snr_rangos.doc_count
type: quantitative
axis: {
title: "# of devices"
}
}
```

It's only the calculate on the flattened snr_rangos that does not seem to work

Is there any way to perform a "calculate" on a flattened field?

Thanks so much in advance for your insights!