The Russian central bank has been burning through the country's foreign exchange reserves as investors sell the ruble, driving its value ever lower.

Figures released by the central bank on Friday showed that it spent almost $13 billion buying rubles from the market in an attempt to stem the collapse of the currency over recent weeks.

Yet despite its intervention, the ruble continued to slide against both the dollar and the euro:

Ruble versus the dollar. Bloomberg



Ruble versus the euro. Bloomberg



The falls have forced the central bank to shift its target trading range against the euro and dollar 37 times since the start of the month. On Monday, Russian central bank head Elvira Nabiullina appeared to concede defeat, saying that if markets continued to turn against the ruble further, currency interventions would not "be able to restrain them."

And here's the chart that explains the sudden investor panic about Russia — the oil price is plunging:

EIA



Brent crude oil touched $115 a barrel in June but has since dropped to a low of $86 a barrel on Tuesday. The plunge has been driven by increased supply because of the US shale oil boom and record production from Russia, as well as slowing demand from a decelerating Asia (especially China) and renewed economic problems in the eurozone.

Oil accounts for almost half of Russia's export income and about 30% of the country's GDP. Where goes oil, goes the ruble, in other words. High oil prices have allowed the country to grow at an average of almost 7% per year since the start of the new millennium, but that is now forecast to slow to 0.4% this year and between 0.9% and 1.1% in 2015, according to the central bank.