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It's 1973 and Britain is in turmoil.

The year begins with a stock market crash and quickly gets worse.

By May, millions are on strike, miners and steel workers, postal workers and train drivers.

Inflation is surging and whole industries are on the brink of collapse.

But away from all of the societal unrest, unmentioned in the news, something else is going on, something subtler and stranger, something that remained invisible until many years later.

In front rooms and hospitals across the country, an unusually high number of baby boys were being born.

We're not just talking about a few extra baby boys here. In 1973 to 74, the ratio of baby boys to baby girls born in England and Wales was higher than at any other time in the 20th century.

What on earth was going on?

I'm Hannah Fry, a mathematician who studies patterns in human behaviour,

and for BBC Radio 4, this is uncharted.

Tales of Data and Discovery.

This is a series about how graphs can help you to see the invisible,

about how plots can be rich with hidden depths and unheard stories,

and about how sometimes if you know where to look,

there is mystery and drama and intrigue to be found,

all concealed within a few lines on a page.

I was researching my book Sex by Numbers, and I plotted out the sex ratio statistics from 1838 to 2012.

And as far as a note, nobody had ever plotted that before.

I've never, ever seen it.

It's just boring numbers in a big spreadsheet.

But when you draw it, it is one of those graphs, one of those delicious graphs,

...