Bitcoin hasn't received the kind of hype in 2015 that it has in previous years. But outside the spotlight, the Bitcoin economy has continued to grow slowly but steadily. And in the last few weeks, the value of Bitcoin has soared. One bitcoin cost $236 at the beginning of October. Now, five weeks later, it costs $426.

No one is sure why the currency has had such a dramatic surge. But it's at least partly a sign of growing investor confidence in the growth of the underlying Bitcoin economy. Here are five charts that show Bitcoin's progress.

1) Bitcoin's price is at its highest level of the year

One bitcoin was worth around $320 at the start of 2015, and it was below that level for most of the year. But last Friday, it rose above $320, and then shot up to more than $420 in the last couple of days.

To be sure, this is far from the currency's all-time record: One bitcoin was worth more than $1,000 in late 2013, and it lost 60 percent of its value in 2014. But the past few weeks' boom suggests that after months of growing pessimism, Bitcoin speculators are becoming more optimistic again.

2) People are performing more Bitcoin transactions than ever

The best way to estimate Bitcoin's long-term growth isn't by its price — which is hugely affected by speculation and media hype — but by the volume of transactions on the Bitcoin network. Over the long run, a growing volume of Bitcoin transactions is a strong signal that people are finding the network more and more useful. And this year we've seen steady — though not spectacular — growth in the number of bitcoin transactions.

This growing transaction volume is a healthy sign for Bitcoin, but it's also a potential problem, because right now the network has an artificial limit on the total number of transactions it can handle. The network will hit this limit in a couple of years if changes aren't made to the Bitcoin software — and the Bitcoin community has yet to reach an agreement about what to do about this.

3) Venture capitalists continue to invest in Bitcoin

Bitcoin hasn't received nearly as much attention from the media this year as it did in 2013, but Bitcoin-based startups continue to be of great interest to venture capitalists. With two months remaining in the year, there has already been $468 million invested in Bitcoin startups — more investment than in any previous year. Bitcoin has yet to break into the mainstream, but a lot of investors are betting it will do so soon.

4) Bitcoin is becoming increasingly international

This chart from the Bitcoin startup Bitpay shows how its business has grown in various parts of the world. Bitpay helps merchants accept Bitcoin payments. Like most Bitcoin startups, Bitpay is based in the United States, so its business was originally skewed toward the US. But recently that's been changing. Europe zoomed ahead of the United States in the first half of 2015, and Latin America has seen rapid growth.

This might be because in many ways the US is the least promising market for Bitcoin. One of the big promises of the technology is that Bitcoin could serve as a global standard, allowing people in different countries to interact. That's not a very strong selling proposition in the United States, which is a single integrated market where people tend to all use the same payment methods.

But Bitpay says that despite the euro, "traditional payments systems in Europe are still very localized and disjointed throughout the continent." The same is true in Latin America. In areas where people are used to juggling many different payment methods, adding Bitcoin to the list is an easier sell.

5) The number of Bitcoin ATMs is growing

A Bitcoin ATM looks much like a conventional ATM. It allows people to insert dollars (or other local currencies) and get bitcoins in exchange. Some machines also allow the opposite: Customers can send bitcoins to the machine and get cash.

This provides a key bit of infrastructure for the Bitcoin ecosystem, because it means that anyone who wants some bitcoins can get them easily. Among other applications, these machines provide an alternative to conventional money-transfer services like Western Union: Someone can put cash into an ATM in New York and send bitcoins to a family member in Argentina or Thailand, who can then use another machine to get out the local currency.

However, right now Bitcoin ATMs are too expensive for this to be practical. Average fees are around 5 percent on each end of the transaction, so using them as a Bitcoin-based money transfer service costs about 10 percent — more than a conventional money transfer service. Bitcoin ATM owners will need to bring these fees down if they want the machines to be more widely used.